FRAMINGHAM, Mass. — Over the long, cold winter, Boston has been preparing a ceremony to honor those killed and injured in the bombings at last year’s marathon and to stage a race this year that will be one of the biggest — and, they said Monday, the safest.

The 118th running of the Boston Marathon, scheduled for April 21, has drawn a huge field of about 36,000 runners, which is capacity for the course and 9,000 more than last year. The runners include thousands who were forced to stop last year after the explosions and thousands more who want to show their solidarity with Boston. (The record number was set in 1996 at the marathon’s centennial celebration, when 38,708 runners entered.)

At least one million spectators, twice the usual crowd, are expected to gather along the 26.2-mile course, many of them at the finish line on Boylston Street, where two bombs exploded, killing three people and injuring 260 others.

The security challenge is immense, in part because the event is spread across eight cities and towns along a route lined by spectators on both sides.

More than 3,500 police officers, twice last year’s number, will be deployed, public safety officials said here Monday at a news conference outlining their security plans. Those plans include plainclothes officers, private security contractors, numerous security checkpoints with metal detectors, bomb-sniffing dogs and hundreds of surveillance cameras.

Spectators are being encouraged to carry their belongings in clear plastic bags to speed up security screening and not to carry backpacks or coolers, wear vests with pockets, or bring baby strollers. Fanny packs are allowed if they measure smaller than 5 inches by 15 inches by 5 inches.

Public safety officials said they had met with security personnel around the world in preparation for this year’s marathon, including several sessions with the New York City Police Department and security officials in London.

“We have to get this right 110 percent of the time; the bad guys only have to get lucky once,” said Kieran L. Ramsey, assistant special agent in charge of the F.B.I.’s Boston office.

Despite the intense security upgrades, officials said they did not want Boston to appear to be a police state and they were trying to retain the festive and traditional character of the event, the oldest continuously run marathon in the world.

“We are confident that the overall experience of runners and spectators will not be impacted and all will enjoy a fun, festive and family-oriented day,” Kurt N. Schwartz, director of the Massachusetts Emergency Management Agency, said.

The Boston Athletic Association had said that unregistered runners, known as “bandits,” who jump into the race along the route, would be banned this year. But Mr. Schwartz demurred when asked about a ban, saying instead that police would respond to them on a case-by-case basis.

The marathon is held on Patriots’ Day, which falls on the third Monday in April and commemorates the Battles of Lexington and Concord in 1775. It is celebrated in only a few states; in Massachusetts, it is a school holiday, which makes the marathon all the more festive.

“I want to encourage everyone to come out,” said Col. Timothy P. Alben, superintendent of the Massachusetts State Police. “This is the opportunity to show the resiliency of the American public, to celebrate Patriots’ Day, school vacation. We’ve got a Red Sox game at Fenway Park that day. I’d love to see people come out.”

On April 15, the date of last year’s marathon, a procession of hundreds of people, including survivors of the explosions, families of the dead and emergency medical workers, will walk down Boylston Street and in front of the Boston Public Library to the finish line.

Wreaths will be laid at the sites of the two explosions. At 2:49, when the first bomb went off, churches throughout Boston will toll their bells, sounds that officials said would signal the region’s mourning and its renewal.