"We're taking back our race," he said. "We're taking back the finish line."
A total of 35,755 athletes were registered to run, the second-largest field in its history, with many coming to show support for the event and the city that was traumatised by the attack.
"I can't imagine the number of emotions that are going to be there," said Katie O'Donnell, who was stopped 1.6km from the end last year. "I think I'm going to start crying at the starting line, and I'm not sure I'll stop until I cross the finish line."
Buses bearing the message "Boston Strong" dropped off runners at the starting line in Hopkinton.
Among the signs lining the end of the route was one paying tribute to 8-year-old Martin Richard, the youngest of those killed in the bombing.
"No more hurting people. Peace," read the sign. A photograph of Martin holding a poster he made for school with those words was published after his death.
"I showed up, I'm back, and I am going to finish what I didn't finish last year," said Mary Cunningham, 50, of St Petersburg, Florida.
Sabrina Dello Russo, 38, of South Boston, was running her first marathon for a good friend, Roseann Sdoia, who lost her right leg in the bombing.
"She is my inspiration from day one last year when I saw her in the ICU. Every run I do, she is in the back of my head, and she will be keeping me going today," Dello Russo said.
Police set up checkpoints along the marathon route to examine backpacks, particularly outside subway station exits. And runners had to use clear plastic bags for their belongings.
More than 100 cameras were installed along the route, and 50 observation points were set up around the finish line to monitor the crowd.
Runner Scott Weisberg, 44, from Birmingham, Alabama, said he had trouble sleeping the night before.
"With everything that happened last year, I can't stop worrying about it happening again. I know the chances are slim to none, but I can't help having a nervous pit in my stomach," Weisberg said.
Race organisers expanded the field to make room for more than 5000 runners who were still on the course last year at the time of the explosions.
Kenya's Rita Jeptoo won the women's race in a record 2 hours, 18 minutes, 57 seconds, defending a championship from last year. She had been hoping this year for a title she could enjoy.
"It was very difficult to be happy. People were injured and children died," she had said of last year's marathon. "If I'm going to win again, I hope I can be happier and to show people, like I was supposed to last year."
