Eyewitnesses at the Boston Marathon described the ground shaking beneath their feet, smoke and "a lot of chaos" this afternoon when explosions killed three people and injured more than 130.
Rachel Sibley, 22, was 50 feet from the finish line waiting for a friend to cross.
"All of a sudden I heard this bang that sounded like a cannon," she said. "You could see people looking up at the sky like there were fireworks, like a celebratory bang. The whole crowd waited for a moment, and then the second one went off. It was terrifying and absolute chaos.
"Everyone needed to get out of there," said Sibley. "I was ready to just start running, otherwise I'd be tackled. You could see the panic in people's faces.
"People started screaming and yelling at each other, trying to find friends and family members," she said. "People started running away from the finish line. There were sirens filling the streets and heading back to the finish line. It was absolutely terrifying."
One doctor in a medical tent about 150 yards from the explosion said he was immediately mobilized.
"We all went running over there and started to bring people into the medical tent," he said. "It was not good. Very bad. Like a war zone. 9/11 immediately came to mind."
He said the subways were immediately shut down and people were walking out of the area.
One eyewitness, Joe Conway, said he saw a giant cloud of gray smoke and ran to the food court at the Prudential Center.
"People were running out and didn't know what was going on," he told ABC News. "I had a baseball hat on and could feel the concussion. The baseball hat fell off my head."
Amanda Fahkkredine, 25, had just walked down into the Arlington subway station a few blocks from the finish line when the bombs went off. As two trains were pulling into the station, she heard it.
"You heard this huge noise and a rumble, and then two T transit police started yelling at everyone to get out of the station," Fahkkredine said. "They didn't seem to know what was going on."
Marathoners began hobbling up the subway stairs. Fahkkredine asked transit police what happened, but they told her they didn't know -- that everyone just had to get out.
"We heard it, and we no idea if there was a train crash or car accident," she said. "It wasn't like anything that I've ever heard or felt before. It was like an earthquake sounding like a car crash."
Back above ground, officers told pedestrians to stay away from landmarks.
Fahkkredine finally got word from her boyfriend that it was a bomb. Although Fahkkredine is not a Boston native, she graduated from Boston University in 2010 and calls the city home.
"I can't believe that just happened," she said. "It's shocking."
The first blast was reported at 2:42 p.m. near the finish line medical tent. By 3:15, authorities said a second device was reported at Saint James Avenue and Trinity Place.
The race was stopped about 3:28 p.m. EMS was tagging everyone affected with red wristbands and beginning transports to Boston hospitals.
A controlled explosion occurred at 4 p.m.
"People were running and screaming and crying in the area," college student Dan Lamariello told ABC News. "There was trampling, running It was a very scary scene."
Racer Jill Elaine Czarnik, 24, of Chicago, finished just 20 minutes after the explosion went off. She was standing in the hospitality area about a quarter mile from the finish line.
"I was in the first wave and I think most people [when the explosions went off] were in the second wave," said Czarnik, who didn't hear the explosion. "It's kind of like a movie scene. ... I think everyone is just kind of like in shock. ... I still feel a little shaky, but I feel safe now. ... It's just very weird ... because you don't know what it is and you're just very delusional because you just ran a marathon."
Phone lines were too busy for many people in the area to reach friends and relatives.
"They're saying just send texts and not to make phone calls so people can reach their loved ones," said Czarnik.
Dan Lau of Boston, a 25-year-old engineer running his sixth marathon, said he was three or four blocks away from the chaos, crossing the finish line about 20 minutes earlier.
"It sounded like something from out of a movie," he said.
"Initially, folks were running towards a narrow, fenced-in exit spilling over to Newbury, but several bystanders diffused the panic and calmed us down," he said. "My first impression was either a truck hit a building or a subway train crashed. Then, when the second went off, my friend and I knew it was more serious. My friend immediately thought it was a bomb. To quote him, he said, 'That's definitely not thunder.'"
Runners further back were stopped by authorities. Josh Crary, one of 40 blind or visually impaired runners, had to stop at mile 20 in suburban Newton, west of the city.
"We stopped after my sister gave me a phone call," said Crary, a 27-year-old from Barnstead, N.H. "She said there had been an explosion and people were running away. Ten minutes later, the race was shut down."
His sister was in a VIP booth at the finish line. Crary said he and his guide were taken in by neighbors.
"Some folks took us into their home," he said. "It's almost impossible to call people because cell phones are busy. Hopefully, this is some sort of freak accident or explosion and not some other terrible scenario."
A Google document circulated on which Bostonians listed their contact information and offered up places for marathon runners to stay. As of three hours after the explosions, 708 people had offered up their homes.
As for not being able to finish the race, Crary said, "I could care less about the finish. There are much bigger things to worry about."
ABC News' Lauren Effron, Liz Neporent, Susanna Kim and Kelley Robinson contributed to this report.