Runners are starting the 5K Yulefest race at the starting line on Massachusetts Avenue. The Yulefest race was a 5K run across Harvard Square in which participants were encouraged to wear holiday costumes.

Over a thousand people dressed as Santa Clauses, elves, and reindeer descended upon Harvard Square for the third annual Yulefest race on Sunday morning.

The run, one of five annual 5k races in Cambridge, kicked off at the intersection of Holyoke Street and Massachusetts Avenue and was followed by a post-race block party on Brattle Street.

Many of the participants in the race, which included students and residents from the Boston and Cambridge area, said that they entered the 5k largely for celebratory, rather than competitive, reasons.

John Jantz, a fifth-year student at Northeastern University and a former track and cross country runner, said that even though he ended his racing career, he decided to join the race after hearing that his friends had enjoyed previous Yulefest runs.

“I thought it would be a fun Sunday morning to get out the door and get started early,” said Jantz. “I’m purposely running really slow today because my racing career is done.... I’m just running for fun today.”

Two runners decked out in matching kilts said that they participated in the race after enjoying previous Cambridge 5k races and after-parties. One of them added that the festive themes always made for a good time.

Despite a light drizzle early on, onlookers and race volunteers cheered on the runners throughout the 5k. Once participants completed the race, they stayed around to encourage fellow runners, chat with friends, and head to the after-party, which was sponsored by four local brewers. A few runners played drums at the finish line.

One woman, who came to Yulefest for the second year to support her friends who were running, said that she is always impressed by the creativity of runners’ costumes. One of the most memorable costumes, she said, was a Santa without pants on who was wearing a speedo underneath his Santa coat.

Event organizers awarded “Best Costume” prizes to individual runners in addition to prizes for the fastest female and male runners and for the fastest teams. Benefits from the race went to Cambridge Family and Children’s Services.