The first wave of the 36,000 runners participating in the 118th Boston Marathon crossed the starting line under tight security on Monday, a year after a deadly bombing disrupted the race.
A moment of silence was held to honour the victims before the athletes, many of whom vowed to run in defiance of the attack, set off to complete the 42-kilometer course.
In the women's field, Shalane Flanagan of the United States was leading 1 hour 30 minutes into the race. Ethiopians Buzunesh Deba and Mare Dibaba and defending women's champion Rita Jeptoo of Kenya followed close behind.
In the men's elite group, Meb Keflezighi and Josphat Boit of the US were in the lead after passing the 19-kilometer mark.
The men's elite field also includes defending men's champion Lelisa Desisa of Ethiopia and Dennis Kimetto of Kenya, who won the Chicago Marathon in October.
The added security for this year's race includes 3,500 security personnel and 3,500 uniformed Boston police officers, 450 state police and 600 members of the National Guard, according to the Boston Athletic association's website.
The number of officers protecting the course is double the number from last year. Race organizers also have set up temporary surveillance cameras along the route.
Three people died, dozens more were maimed and more than 260 were injured on April 15, 2013, when two bombs made of pressure cookers concealed in backpacks were detonated at the finish line.
©2014 Deutsche Presse-Agentur GmbH (Hamburg, Germany)