An eight-year-old boy has been named by US media as one of three people killed when two bombs ripped through crowds during the Boston Marathon.
The Boston Globe newspaper identified the youngster as Martin Richard, who lives in the city.
He was reportedly waiting to give his father, who was running in the race, a hug at the finish line when the blasts happened.
The boy's sister lost a leg in the attack and his mother was also seriously injured, according to reports.
A candle was left on the steps of the family home in the Dorchester district and the word "peace" was scrawled in chalk on the sidewalk.
The US is on alert for further attacks as the FBI leads a terror investigation into the bombings, which left more than 140 people injured - many seriously - in the heart of Boston.
Earlier, people working at the New York Stock Exchange started the day with a minute's silence out of respect for the victims.
Spectators' cheers turned to screams as the bombs went off within seconds of each other and about 100m (330ft) apart on the same street, blowing out windows and sending smoke and debris into the air.
Emergency workers tore down fencing and carried away seriously injured men and women amid scenes of panic and confusion.
Of the 144 reported injured, 17 are in a critical condition.
Harrowing stories have emerged after doctors confirmed "several amputations" were performed on survivors. Others had limbs torn of by blast forces.
Liz Norden, a mother of five, told the Boston Globe how two of her sons had each lost a leg in the blast.
Both had gone to Boylston Street to see a friend finish the race.
"Ma I'm hurt real bad," Ms Norden quoted one of her sons as telling her in a phone call from the ambulance.
There have been no immediate claims of responsibility for the attack, the most serious in the US since the 9/11 World Trade Centre atrocity. Al Qaeda-linked groups and militant white extremists have attacked targets in America in the past.
Barack Obama vowed to find and punish those responsible, as a senior White House official said the attacks were being treated as an act of terrorism.
Mr Obama said officials "still do not know who did this or why".
But he vowed: "We will find out who did this. We'll find out why they did this. Any responsible individuals, any responsible groups, will feel the full weight of justice."
A European security official with knowledge of the US investigation said initial evidence suggests suicide bombers were not involved.
The marathon is held every year on Patriots' Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution in 1775.
The explosions happened four hours into the race and about two hours after the men's winner had crossed the line, as amateur runners were reaching the finish.
More than 17,000 competitors had completed the race by the time the blasts struck.
TV helicopter footage showed blood on the ground and the desperate efforts of rescuers in the popular shopping and tourist area known as the Back Bay.
A woman near the second bomb, Brighid Wall, 35, said people had frozen, 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."
She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed and bleeding from his head. Another person was on the ground covered in blood and not moving.
"My ears are zinging", she said. "Their ears are zinging. It was so forceful. It knocked us to the ground."
Bill Iffrig, a runner who was filmed falling to the ground as the first blast went off, said "the shockwave must have hit me. My legs felt like noodles". He was able to walk away and speak to reporters at the scene.
Roupen Bastajian, 35, a state police officer from Rhode Island, had just finished the race when he heard the blasts.
"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."
One British runner, Abi Griffiths, 34, from London, told Sky News the scene was in a "state of chaos" .
She said: "Police were everywhere, we were being evacuated out of the area and it was really eerie."
A senior US intelligence official said another two unexploded bombs were found and disarmed near the end of the 26.2mile (42km) route.
No one has been arrested, although officers searched an apartment in the Boston suburb of Revere as part of the investigation.
A no-fly zone was also put in place over the city as security was tightened and flights bound for Boston's Logan International Airport were briefly held up at other airports.
Boston Police Commissioner Edward Davis said the authorities had received "no specific intelligence that anything was going to happen".
At Massachusetts General Hospital, Alisdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
Police and doctors quoted by US media said ball bearings had been packed into the injuries, causing horrific injuries.
British police are now reviewing security plans for this Sunday's London Marathon - the next major international marathon.
The London race's chief executive, Nick Bitel, said it was "a very sad day for athletics and for our friends in marathon running".
More than 25,000 people were registered as taking part in the race, 374 of whom were British. There were also 108 Irish athletes.
The British Foreign Office has said it is not aware of any British nationals who have been injured but that it will continue to monitor the situation.
There were reports of a third blast at the JFK library a few miles away but that was later confirmed as a fire that was believed to be unrelated to the blasts.
Prime Minister David Cameron wrote on Twitter: "The scenes from Boston are shocking and horrific - my thoughts are with all those who have been affected."
Boston officials said it would "not be business as usual" in the city, with random checks of backpacks and bags on public transport. Security has also been stepped up in Washington and New York.
Russian sports officials have also announced increased security planning for the Sochi 2014 Winter Games in the wake of the explosions.
:: The British Foreign Office said friends and relatives concerned about British nationals in Boston can call the Boston Mayor's Office emergency hotline on 00 1 617 635 4500.
:: In the US, concerned relatives can call 617 635 4500, and anybody with information about the blasts should call 1 800 494 tips.