At least two people are dead, including an eight-year-old child, and an estimated 110 injured after two bombs went off in the bustling downtown area on Monday.
Eight people are in critical condition and eight children have been injured with one of the youngest reported as a two-year-old child. Mass General said four of those injured were undergoing leg amputations.
Footage from camera phones, trained on the finish line on Boylston Street, shows a huge, fiery blast ripping through barriers, throwing runners and spectators to the ground at 2.50pm (ET).
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Terror on the streets: Two explosions took place at the Boston Marathon on Monday at 2.50pm as runners crossed the finish line
Clouds of smoke rose in the air and pains of glass were heard shattering.
Seconds later, a second, thunderous explosion is heard as people run in all directions screaming and yelling for help.
Police officers are seen drawing their weapons and heading towards the site of the blast.
Stands for spectators are torn away by police and bystanders as emergency crews, who had been on hand for runners, try to reach the injured.
The FBI is now classifying this afternoon's horrific bombings as a terrorist attack. The attacks took place on Patriots' Day, an annual holiday in Massachusetts.
Hotel owner Mark Hagopian told MailOnline last night of the carnage he had witnessed, 35 yards from the explosion.
Mr Hagopian said: 'It was chaos - blood and limbs everywhere. There was a man who had lost a leg, another had lost both.
'There were seven, eight, nine people lying on the ground. They appeared to be dead.
'Marathon runners were tearing off their shorts to use as tourniquets to help the injured.'
Mr Hagopian was inside the Charlesmark Hotel, on Boylston Street, when the first bomb went off. As he rushed outside, the second exploded.
'There was blood everywhere, along with severed limbs. It was just awful,' he said.
'The second bomb came about ten seconds after the first. People were knocked off their feet by the force of it.'
The hotel owner, 50, videotaped the aftermath, showing a man lying apparently unconscious on the pavement covered in blood.
Another man can be seen tearing off his red t-shirt to use as a tourniquet as police officers rush to help.
One person is overheard saying: 'What the **** happened, a bomb?'
Another man, wearing a David Beckham soccer shirt, is seen running around. The area around the hotel was evacuated. Mark and his guests gathered in a restaurant two blocks away waiting for police instructions.
Mr Hagopian’s mother Marcia Scott-Harrison, who was evacuated from nearby Commonwealth Avenue, said: 'Mark is in total shock but at least he wasn’t hit. His hotel is just yards from the finish line. A man sitting outside the hotel had his leg blown off. It is awful.
'I heard both explosions, then a dozen cops came running up Commonwealth Avenue. They were shouting for everyone to get off the street.
'There were young girls crying, I pulled three into my apartment building. It was chaos. No one knew what was happening.'