BOSTON (AP) — The ladies of Wellesley College will still be allowed to plant kisses on passing runners and crowds will still flock to the finish line at the 118th Boston Marathon.

But a year after deadly twin explosions turned the race’s festive final dash into a scene of devastation, police and organizers of the world’s oldest annual marathon find themselves balancing security with its traditionally festive atmosphere.

“We want this event to be what it always has been, and that’s a special day for the city, the best marathon in the world.” said Boston Police Commissioner William Evans. “We don’t want to change that.”

But the bombing has affected nearly every detail of planning for the April 21 race as police work to beef up security along the 26.2-mile course. Participants who favor flamboyant costumes will have to tone it down, and unregistered “bandit” runners will no longer be permitted. Runners won’t be allowed to stow personal items in a backpack but instead will be provided with clear plastic bags to hold a change of clothes that will be bused to the finish line.

And spectators will encounter security checkpoints along with hundreds more officers, bomb-sniffing dogs and other security measures that haven’t yet been disclosed. Public safety officials are expected to release more details Monday.

“We are going to have a lot of security, but we want to do it more low-key so we don’t alarm people who might want to come,” Evans said.

Last year, authorities say two brothers built bombs out of pressure cookers, carried them to the finish line in backpacks and set them off. The bombs tore into the crowd, killing three people and injuring more than 260.

This year, spectators are being strongly discouraged from bringing backpacks, coolers, quilts and other bulky items.

“People have to realize that if they are going to come, they are going to be subjected to the possibility of searches,” Evans said.

Police believe the expanded field and the “Boston Strong” solidarity that followed the attack could attract up to a million spectators — about twice the usual number. The Boston Athletic Association accepted 9,000 extra runners, including about 5,000 who were forced to stop last year and thousands more who want to run to pay tribute to the victims of the attack.

Security experts say the marathon presents a huge challenge.

“You need the convergence of luck and timing and fabulous intelligence, great police work. You need the stars and the moon and everything to line up together in order for it to be 100 percent safe,” said Robert Tucker, chief executive officer of T&M Protection Resources, a New York-based security firm.

Tucker said police would likely focus more resources on the finish line, where throngs of people gather, and ban any items that potentially add risk.

“They should forget about convenience and figure out what the real things are that they need to do to prevent something like what happened last year,” he said.

A list of new rules for runners includes a limit on the size of water bottles and restrictions on costumes — nothing bulky or covering the face.

Along the route, there will be more surveillance cameras and more police, including 400 armed military police from the National Guard, an unspecified number of undercover officers and hundreds more uniformed officers. Security will be especially tight at the start of the race in Hopkinton and at the finish line, where the largest crowds gather.

Police have gone out of state for special training on how to detect hidden explosives and to look for characteristics of someone who might be carrying a bomb.

“Spectators should expect that they may be channeled through security screening points at different places as they get closer to the course,” said Kurt Schwartz, director of the state Emergency Management Agency.

Police in each of the eight cities and towns along the course have been beefing up their own security plans and coordinating efforts with one another.

In Hopkinton, town officials are scrutinizing vendors more carefully. When they realized that some have sold toy guns in past years, they banned their sale.

“We just felt that in this atmosphere, for this event, it was not appropriate,” said John Mosher, chairman of Hopkinton’s board of selectmen.

In Ashland, police are trying to strike a balance between safety and maintaining the closeness residents have with the runners.

“It’s an open event, so I think it would be impractical to tell people who are standing on the roadway in front of their houses that they can’t have a backpack or container on their front lawn,” said Police Chief Craig Davis. “There’s obviously going to be enhanced security, checking, examining those areas and containers.”

In Wellesley, the marathon’s halfway point, police will have more officers along the course, but they don’t plan to try to stop a favorite marathon tradition: a line of Wellesley College students kissing and hugging runners.

“We’re not planning on raining on their parade at all,” said Deputy Police Chief Jack Pilecki. “We just want to keep them safe — and everyone else.”