The elite women started at 9:32 a.m., the elite men at 10 a.m. By then, the streets were jammed with spectators and thousands of volunteers filling paper cups with water and Gatorade for thirsty marathoners.
People climbed lampposts to get better vantage points, held out their hands to give high-fives to passing runners, and greeted the police, National Guard troops and other security forces on patrol.
Signs, many of them posted the evening before, dotted trees and lampposts along the way.
Some were inspiring. "Run, Mom, run!" "Anne, don't give up!"
Others were designed to make weary runners smile. "Chafe now, brag forever." "Run like you stole something."
With the elite runners underway, it was time for the rest of the pack, nearly 30,000 strong, to hit the road.
Michelle Jacobsen didn't expect to get emotional, but she began to cry as she jostled her way to the start point, passed beneath a sign reading "Good luck runners," and started to run. "Just that the city comes together for this," she said at the finish line, explaining the flood of feelings that came over her from her first steps.
By mile 23, she was tempted to walk, but she kept running. "I kind of kicked it forward," the Newport Beach woman said.
After finishing, Jacobsen stood at the bomb site, taking in the crowd. She had hoped to stand here last year after running her first Boston Marathon. Instead, she went to shower and never made it back because of the blasts.
This year, Jacobsen was determined to enjoy the moment as long as possible. She planned to head to the airport, still in her running clothes, later Monday and hop on a plane to get back to L.A. in time for work Tuesday.
Mike Poitras was also at the finish line, with his two sons. His shaved head was painted blue and yellow, with the words "Boston Strong" on one side. He wore a Boston Strong T-shirt and carried a sign that read: "No More Hurting People. Peace."
The inspiration for his sign came from bombing victim Martin Richard, 8, who made a similar sign after the 2012 Trayvon Martin killing.
"We always go to the finish line," Poitras said, "but this year, more than ever."
Times staff writers Nathan Fenno and Stacey Leasca contributed to this report.