The temperature in Boston on Monday is forecast to hit 86 F (30.5 C) by early afternoon, a record for this date and about 30 F above normal, making conditions especially trying for slower runners among the 27,000 entrants, who will be out on the 26.2-mile (42-km) course for many hours.

"We have determined that the race will occur in a 'red zone' which is considered an increased risk but acceptable for high-level elite runners. However, it is not considered safe for unfit and novice runners," the Boston Athletic Association said in a note to entrants.

Marathon organizers have urged runners who did not gain entry through a competitive age-graded time - the several thousand entrants associated with charities, vendors, sponsors, local municipalities and other groups - should not start the race. Their coveted entries will be held over to 2013, the BAA said.

Pierre d'Hemecourt, the BAA's co-medical director, said participants should be vigilant about headaches, dizziness, confusion, fatigue, nausea or vomiting, which can be symptoms of heat exhaustion or heat stroke.

"If you have underlying medical problems, such as cardiac disease or respiratory disease, think about not running," said d'Hemecourt. "If you have a cough or cold, or had recent gastro-enteritis, don't run."

Organizers are boosting supplies of water and ice along the course, and have arranged additional ambulances and medical sweep buses. Red Cross stations will add workers along the race route, and fire departments will have spray hoses in places for runners to cool down.

The men's elite field is led by Kenyan Geoffrey Mutai, who in 2011 ran the fastest marathon in history in Boston, 2 :03:02, and went on to win the New York Marathon in the fall to complete a rare double.

But nine of the men's field own personal marathon bests under 2:07, making it possibly the strongest field ever assembled.

Caroline Kilel, also of Kenya, will defend her 2011 women's title. One elite Ethiopian runner, Buzunesh Deba, dropped out of the race on Sunday because of a foot injury.

Top U.S. runners are sitting out this year's Boston race ahead of the London Olympics marathon races in August.

Marathon competitors will start in several waves. Elite women runners will start at 9:32 a.m. EDT followed by elite men at 10:00 a.m. EDT.

The final wave of runners will leave Hopkinton, west of Boston, at 10:40 a.m. Temperatures are forecast to be over 70 F (21 C) by then.

(Reporting By Ros Krasny; Editing by Eric Beech)

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