The United States' Meb Keflezighi, 38, crossed the finish line to become the first American man to win the event since 1983, race officials tweeted. His unofficial time is 2:08:37, according to the race's Facebook page.
This year's race has 9,000 more runners than last year's, and an unprecedented crowd was expected to gather, officials have said.
Officials have worked hard to make sure security is very tight Monday. Backpacks are not allowed on the course this year, said Kurt Schwartz, director of the Massachusetts Emergency Management Agency.
Costumes that cover the face, containers with more than 1 liter of liquid, and bulky clothes such as vests with pockets are also prohibited.
Runner's World is carrying a list of the men and women who are expected to perform best.
Information via CNN