BOSTON – The 100th running of the Boston Marathon could attract such huge crowds that some organizers fear just starting the 26.2-mile circuit could be as unsafe as it is unwieldy.

The marathon’s technical coordinator, Dave McGillivray, estimated that the race can accommodate a maximum of about 37,500 runners. But Hopkinton Marathon Committee chairman Robert Phipps said that as many as 54,000 people could show up to run the April race.