"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.