The enhanced police presence is part of a beefed-up security plan detailed Monday by public safety officials as they prepare for the April 21 marathon.
Spectators who plan to attend the marathon are being strongly discouraged from bringing backpacks, rolling bags, coolers and other large items, and are instead being asked to carry personal items in clear plastic bags. Anyone who does bring a bulky bag will be subject to search, officials said during a news conference at the Massachusetts Emergency Management Agency's bunker in Framingham.
The new guidelines for spectators are similar to rules for runners made public several weeks ago by the Boston Athletic Association.
Authorities said they sought to strike a balance between keeping the traditional feel and character of the marathon and tightening security in response to last year's deadly terror attack.
"We are confident that the overall experience of runners and spectators will not be impacted, and that all will enjoy a fun, festive and family-oriented day," said Kurt Schwartz, director of the Massachusetts Emergency Management Agency.
Schwartz said police are looking to spectators to be especially vigilant and report any suspicious items or activity.
Like runners, spectators are also being told they cannot bring containers with more than 1 liter of liquid and cannot wear bulky costumes or anything that covers their faces.
Unregistered runners known as "bandits" will be prohibited this year. Officials said the move was necessary because or security concerns and because organizers are expecting 9,000 additional runners this year - bringing the total field to 36,000 - and double the typical number of spectators for a crowd of up to one million.
"This course is at capacity this year and it's just common sense" for bandits to stay off the course, Schwartz said.
Police and other public safety officials from the eight cities and towns along the marathon course have been meeting for months to come up with a plan to beef up security following last year's deadly attack. Two brothers are suspected of building homemade pressure-cooker bombs and placing them in backpacks near the finish line.
Dzhokhar Tsarnaev has pleaded not guilty to 30 federal charges and faces the possibility of the death penalty. His brother, Tamerlan Tsarnaev, died following a shootout with police several days after the bombing.
Kieran Ramsey, an assistant special agent in charge of the FBI's Boston office, said the agency has "no specific intelligence indicating there is a threat to this year's marathon."
"At this point, we don't have one, nor do we anticipate it," Ramsey said.
Ramsey said the FBI and its law enforcement partners have met with officials in other cities that host large scale public events, including London and New York City.
Col. Timothy Alben, commander of the state police, said police will have more than 100 additional security cameras along the route and have also met with business owners to coordinate use of their surveillance cameras, as well. He urged spectators to report anything at all suspicious to police.
"In this world, you never eliminate risk, you never bring it down to zero ... but we are working very hard at reducing that risk level and managing it to the best of our collective abilities," he said.