Ethiopia's Lemi Berhanu Hayle and Atsede Baysa will line up among the world's top distance runners on Monday to defend their Boston Marathon titles, with tens of thousands of spectators expected to line the course amid tight security.
Men's champion Hayle won the 2016 marathon in a tactical 2 hours, 12 minutes and 44 seconds, while women's title holder Baysa won in 2:29:19.
The 121st Boston Marathon will see four-time Olympian Meb Keflezighi make his farewell performance. Keflezighi became the first American to win the Boston Marathon in more than three decades in 2014 with a time of 2:08:37.
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Security will be high along the course, that begins in Hopkinton, Massachusetts, and passes through Boston's suburbs to the finish line on Boylston Street, where a pair of ethnic Chechen brothers in 2013 set off two homemade bombs, killing three people and injuring more than 200.
Tens of thousands of people are expected to pack the streets for Boston's largest sporting event, held on the Patriots Day holiday, which commemorates the start of the American Revolution.
Other top men's contenders include Kenya's Emmanuel Mutai, who won the 2011 London marathon, Ethiopia's Yemane Tsegay, who finished third in Boston in 2016, with the top American contender Galen Rupp of Portland, Oregon, fresh off a bronze in the Olympic marathon in Rio de Janeiro.
In the women's competition, runners to watch include Kenya's Gladys Cherono, who won the 2015 Berlin Marathon, and Edna Kiplagat, who finished second in Chicago last year.
Desiree Linden brings in the fastest time of any of the American women, with her 2:22:38 personal record set when she finished second in Boston in 2011.