By Elizabeth Barber

BOSTON (Reuters) - Boston officials outlined a safety plan on Friday for the 2015 Boston Marathon that would protect runners and spectators while maintaining an upbeat mood two years after an attack at the event killed three people and wounded many others.

Police will limit the crowds near the finish line on Boylston Street, where a pair of ethnic Chechen brothers set off twin pressure-cooker bombs that injured 264 people and set the stage for the most chaotic week in Boston's living memory.

While the number of spectators will be limited along parts of the 26.2-mile course and large bags and parcels will be subject to search, officials said they would preserve the open atmosphere of an event that draws about 500,000 spectators.

"The city will be the same positive environment that people are used to enjoying during the marathon," said Boston Mayor Marty Walsh, describing the impending event on Monday as a "safe and joyful day for everyone."

More than 100 cameras will oversee the route through eastern Massachusetts, from a suburb of Hopkinton to downtown Boston, with 50 observation points near the finish line, said Police Commissioner William Evans, adding: "Don’t leave backpacks laying around."

  Emergency medical services for the race will include two large medical tents near the finish line, 14 ambulances and a 30-bed ambulance bus parked in a downtown Boston park.

This year’s marathon falls during a break in the trial of 21-year-old Dzhokhar Tsarnaev, who was found guilty last week of carrying out the 2013 attack alongside his 26-year-old brother Tamerlan, who died after a gunfight with police four days after the bombing.

Prosecutors are expected to begin arguing one day after the 2015 marathon in the Boston federal courthouse, about a mile from the marathon’s finish line, that Tsarnaev should receive a death sentence for the bombings.

(Editing by Scott Malone and Bernadette Baum)