Boston marathon honours bomb victims
Thousands of marathoners have taken to the streets of Boston, as the city honoured three killed and more than 260 injured in a bomb attack last year.
Dignitaries, spectators and more than 36,000 runners observed a moment of silence before the race began.
The heavy security operation included a ban on rucksacks and screening at checkpoints.
US runner Meb Keflezighi and Rita Jeptoo from Kenya won the men's and women's races.
The BBC's Nick Bryant in Boston says that the marathon had never been run amid so many layers of security, as 500,000 are expected to watch the activities.
Athletes with disabilities were the first competitors to set off, at 08:50 local time (12:50 GMT).
The elite women's race started at 09:32, with the elite men setting off half an hour later, followed by thousands of other runners.
Keflezighi won the men's race, clocking in at 2 hours, 8 minutes and 37 seconds.
He is the first US runner to take the title since 1985.
Jeptoo was the first to cross the finish line in the women's race, marking her third win in the competition.
She finished the course in a record 2 hours, 18 minutes and 57 seconds.
The 26.2-mile (42.2km) route was tightly guarded in a massive mobilisation of law enforcement agencies, including police units, bomb squads and tactical assault teams from other states.
The Boston police department erected 8,000 steel barricades, 1,200 more than last year.
Before the race, runner Katie O'Donnell, who competed in last year's race, said she could not imagine the range of emotions she would feel this 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," she said.
Many people in the city have been wearing "Boston Strong" T-shirts.
Last year's winner of the men's elite race, Ethiopian Lelisa Desisa, is competing again and has met several victims of the blast.
He said the victims had given him inspiration: "We have to look to the future. There has to be a resilience. I am ready to win again."
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.