Thousands of marathoners have taken to the streets of Boston, after the city honoured those killed and injured in a bomb attack last year.
A moment of silence was held ahead of the start before some 36,000 runners set off, 9,000 more than normal.
Officials have mounted a huge security operation that includes a ban on rucksacks and screening at checkpoints.
In last year’s race two bombs exploded near the finish line, killing three people and wounding more than 260.
Last year’s winner of the men’s elite race, Ethiopian Lelisa Desisa, is competing again and has
Running duo Natalie Stavas and her father Joe recall last year’s Boston marathon bombing
Bombing suspect Dzhokhar Tsarnaev, 20, is due to stand trial in November. He has pleaded not guilty to 30 charges, including 17 that carry the death penalty.
Prosecutors allege that he set off two pressure cooker bombs with his older brother Tamerlan, 26, who later died in a police shootout.