Police say at least three people have been killed in the explosions near the finish line of the Boston Marathon.
Police commissioner Ed Davis confirmed the three deaths but provided no details.
The explosions Monday also injured more than 130 people, knocking spectators and at least one runner off their feet.
Some of the victims lost arms and legs. Other injuries included broken bones, shrapnel wounds and ruptured eardrums.
A senior U.S. intelligence official said two other explosive devices were found near the end of the 26.2-mile course.
"They just started bringing people in with no limbs," said runner Tim Davey, of Richmond, Va. He said he and his wife, Lisa, tried to keep their children's eyes shielded from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but "they saw a lot."
"They just kept filling up with more and more casualties," Lisa Davey said. "Most everybody was conscious. They were very dazed."
There was no word on the motive or who may have launched the attack, and police said no suspect was in custody. Authorities in Washington said there was no immediate claim of responsibility.
The twin blasts at the race took place almost simultaneously and about 100 yards apart, tearing limbs off numerous people, knocking spectators and at least one runner off their feet, shattering windows and sending smoke rising over the street.
Local officials react to Boston bombings
Police in Los Angeles, New York City, London and other cities worldwide are stepping up security following explosions at the Boston Marathon.
The Los Angeles County Sheriff's Department has heightened security measures in the aftermath of the explosions.
Sheriff's spokesman Steve Whitmore says the department has opened the Emergency Operations Center, increased patrols for transit and other critical areas including the Los Angeles Dodgers game Monday night.
Whitmore says the department has also increased security around all county buildings and is coordinating with other law enforcement to gather more information about the incident.
The Los Angeles Dodgers declined to comment on heightened security measures at the ballpark, with a spokesman saying only that security is a paramount priority. The Dodgers play the San Diego Padres at the stadium on Monday night.
L.A. Mayor Antonio Villaraigosa's office issued a statement:
"Our thoughts and prayers are with the people in Boston in the aftermath of this senseless tragedy. We are working closely with our law enforcement partners to monitor the situation and stand ready to aid Boston in any way possible. The LAPD and other public safety agencies will remain vigilant at all public and sporting events in Los Angeles."
The Sheriff's Department had yet to receive any suspicious calls or reports.
L.A. Port spokesman Phillip Sanfield said that security has been heightened at the port, but that they don't believe there is a threat to the port complex. Port of Long Beach spokesman Lee Peterson said the Long Beach port is making sure that its security team is aware of what happened on the East Coast.
“Whenever situations like this happen, we monitor and take even a closer look at what might be going on in and around the port,” Peterson said.
L.A. Metro said that there would be heightened security on the public transportation system in the wake of the Boston attacks.
Other California officials take action
"My understanding is that it's a terrorist incident," Sen. Dianne Feinstein told reporters, saying she had been in contact with U.S. intelligence agencies. Feinstein, who chairs the Senate Intelligence Committee, said intelligence officials reported no advance warning that "there was an attack on the way."
California safety officials on Monday called for heightened vigilance and the activation of a statewide threat assessment system in response to explosions at the finish line of the Boston Marathon.
Officials in San Francisco, Los Angeles, Orange County, San Diego and Sacramento were reviewing information from federal authorities involving possible local threats, said Kelly Huston, assistant secretary of the California Emergency Management System.
"At this point we don't see a connection to California, but it's pretty early," Huston said.
Officials were analyzing information coming in from officials in Boston and the federal department of Homeland Security.
The emergency management system was established after the Sept. 11, 2001, terrorist attacks on the World Trade Center and the Pentagon.
California officials were examining if there was "any shred of information coming out ofBoston correlated to something we know about locally, trying to see if there's any connections, and if there is a connection, who needs to know about it," Huston said.
Those questions could lead to adding more police or stepping up security at large events.
No immediate connections were found between the Boston blasts and California, but Huston said, "the sooner we can find connections and then do something about it, the safer we are."
The assessment also involved protecting critical systems in California.
Officials were coordinating with the five regional threat assessment centers in San Francisco, Los Angeles, Orange County, San Diego and Sacramento.
In Sacramento, California Highway Patrol Officer Sean Kennedy, who heads security at the state Capitol, declined to comment on whether security was being increased.
"Security measures are in place to thwart foreign or domestic acts of terrorism," he said.
The San Francisco Police Department is on heightened alert.
"Officers on patrol are being asked to be extra vigilant around critical infrastructures around the city and areas where large crowds gather," said Officer Albie Esparza, a department spokesperson.
The explosions are also causing the department to rethink security logistics for the upcoming San Francisco Marathon and Bay to Breakers foot races.
Airports in Los Angeles, Van Nuys and Ontario were in a heightened state of vigilance with increased patrols to show more police were on duty, said Chief of Airport Police Patrick Gannon.
"We have no indications that suggest there's a nexus from Boston to the Los Angeles airport, but in an overabundance of caution, we have heightened our patrols at all of our airports," Gannon said.
The company that runs Los Angeles International Airport issued a statement saying that security at the airport would be heightened, despite no indication of L.A. being a target.
Orange County Sandra Hutchens said deputies will have an increased presence at sporting and entertainment venues, and other large gathering places. The Angels are scheduled to play baseball in Anaheim on Tuesday and Wednesday nights, and an Anaheim Ducks game is scheduled for Wednesday night.
In San Diego, law enforcement agencies were monitoring developments closely but haven't taken any special measures, authorities said.
There were no major events scheduled in the city.
"Obviously everyone is aware of it," said San Diego County sheriff's Lt. Scott Amos.
Boston residents cautioned to stay inside
Some 23,000 runners took part in the race, one of the world's oldest and most prestigious marathons. One of Boston's biggest annual events, the race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know whether the bombs were planted in mailboxes or trash cans.
He said authorities had received "no specific intelligence that anything was going to happen" at the race.
Federal government responds to Boston bombings
The Federal Aviation Administration barred low-flying aircraft from within 3.5 miles of the site.
President Barack Obama was briefed on the explosions by Homeland Security adviser Lisa Monaco. Obama also told Mayor Tom Menino and Gov. Deval Patrick that his administration would provide whatever support was needed, the White House said.
A fire broke out at the John F. Kennedy Library a few miles away. The police commissioner said it may have been caused by an incendiary device but it didn't appear to be related to the bombings.
What it was like when the explosions went off
"There are people who are really, really bloody," said Laura McLean, a runner from Toronto, who was in the medical tent being treated for dehydration when she was pulled out to make room for victims.
About four hours into the race and two hours after the men's winner crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.
By that point, more than 17,000 of the runners had finished the race, but thousands of others were farther back along the course.
The Boston Police Department said two people were killed. Hospitals reported at least 73 injured, at least eight of them critically.
A senior U.S. intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly.
A woman who was a few feet from the second bomb, Brighid Wall, 35, of Duxbury, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up."
After a minute or so without another explosion, Wall said, she and her family headed to a Starbucks and out the back door through an alley. Around them, the windows off the bars and restaurants were blown out.
She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood coming down his head. Another person was on the ground covered in blood and not moving.
"My ears are zinging. Their ears are zinging. It was so forceful. It knocked us to the ground."
Competitors and race volunteers were crying as they fled the chaos. Authorities went onto the course to carry away the injured while race stragglers were rerouted away from the smoking site.
Roupen Bastajian, a 35-year-old state trooper from Smithfield, R.I., had just finished the race when they put the heat blanket wrap on him and he heard the blasts.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
Smoke rose from the blasts, fluttering through the national flags lining the route. Blood stained the pavement in the popular shopping and tourist area known as the Back Bay.
Cherie Falgoust was waiting for her husband, who was running the race.
"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."
Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.
The Boston Marathon honored the victims of the Newtown, Conn., shooting with a special mile marker in Monday's race.
Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary school.
Explosion at coply twitter.com/Boston_to_a_T/…— Boston to a T (@Boston_to_a_T) April 15, 2013