Marcel Hug of Switzerland has won the men’s push rim wheelchair race, defending his 2015 title.

Leading off this year’s marathon were the mobility impaired runners, which included bombing survivor Adrianne Haslet.

A professional ballroom dancer, Haslet lost a leg in the bombing three years ago. She is one of 21 survivors competing in this year’s race.

It’s her first time running the race since the bombing.

In a special moment last year, Haslet’s two brothers ran the race, and she joined them at the finish line.

On Friday, she threw out the ceremonial first pitch at Fenway Park before the Red Sox played the Toronto Blue Jays.

Haslet isn’t the only survivor to get the honor at Fenway over the weekend. Patrick Downes, who lost his left leg in the 2013 bombings, threw out the first pitch to Red Sox designated hitter David Ortiz on Sunday.

Additionally, Jeff Bauman, who lost both legs in the bombings, is scheduled to throw out the ceremonial first pitch today.

Downes and Haslet both are running to raise money for people with physical disabilities.

In the elite runners’ division, the women’s and men’s champions from last year are back in Caroline Rotich of Kenya and Lelisa Desisa of Ethiopia.

Both are title contenders. However, there’s no clear favorite for either the men’s or women’s races. It could come down to a final sprint on the homestretch on Boylston Street.

Because it’s an Olympic year, the best American marathon runners are focusing on training for Rio in August rather than this race.

The U.S. Olympic marathon trials were in February in Los Angeles. Many of the top Americans are in Boston, although you won’t see them hitting the pavement.

That includes Meb Keflezighi, who was the first American to win the Boston Marathon in 31 years when he did so in 2014, and Shalane Flanagan, a Massachusetts native who has finished twice in the top 10 at the Boston Marathon. Both were at Fenway Park for the ceremonial first pitch on Saturday.

In the women’s wheelchair division, Tatyana McFadden typically dominates the race, winning in 2013, 2014 and 2015.

Around 30,000 people are registered for the race.

The 26.2-mile course starts in Hopkinton and passes through Ashland, Framingham, Natick, Wellesley, Newton and Brookline before reaching the finish line on Boylston Street in Boston.

This year, it is estimated that up to a million spectators may line the streets to watch the marathon.

Because of the 2013 bombings, as well as previous terrorist attacks in Brussels and Paris in the last six months, security was tight Monday.

For the third consecutive year, a number of government agencies and universities offered comprehensive training for public safety personnel supporting the Boston Marathon.

They included the Massachusetts Emergency Management Agency (MEMA), Massachusetts State Police (MSP), Massachusetts Department of Fire Services (DFS), Massachusetts Department of Public Health (DPH),

Boston Police Department, U.S. Department of Homeland Security (DHS), National Bomb Squad Commanders Advisory Board, New Mexico Tech, Texas A&M Engineering (TEEX), and Louisiana State University

More than 500 police officers, firefighters, emergency medical technicians (EMTs) and paramedics attended these training courses leading up to the race, and over 1,800 individuals have been trained over the past two years.