MARATHON WATCH: Runners gathering in Boston

Pepi Bolognese cleans the finish line before the running of the 118th Boston Marathon Monday, April 21, 2014 in Boston. (AP Photo/Robert F. Bukaty)

Thousands of runners are gathering at the starting line of the Boston Marathon in Hopkinton, one year after a pair of homemade pressure-cooker bombs killed three people and wounded more than 260 others.

State and local police officers were everywhere Monday morning, even on the rooftops of some buildings. But rather than a tense situation, everyone appeared relaxed. Some runners even thanked the police officers for making them feel safe.

Near the finish line, spontaneous applause and whoops broke out in the crowd as a group of Boston police officers walked down the center of Boylston Street.

About 36,000 runners have registered for the race — the second-largest field in its history, many of them coming to show support for the event and the city that was shocked by the attack on its signature sporting event.