For Geoffrey Mutai, the bombs that ripped through Boston stripped away some of the innocence and freedom from marathon running.

"Sport is like church, it's not a place where you can take arms," the Kenyan told The Associated Press on Wednesday. "It is a free area where we can stay free and enjoy ourselves with no politics."

But Mutai watched from afar Monday as the scene of one of his great triumphs two years ago turned into one of unimaginable horror.

With three people killed and more than 170 wounded by the twin blasts near the Boston Marathon finish line, Mutai is apprehensive as he prepares to take on the London Marathon on Sunday.

And he fears marathons might never be the same again.

"They have taken our freedom which we normally have in races," Mutai said. "When you are in a race you are relaxed and you are enjoying yourself and free to go anywhere.

"But now there must be watertight security, they cannot be having as many people at the end of the race."

The roar of the crowd along the route will leave Mutai on edge, fearing he is hearing "some sounds of the bomb."

"It will be challenging for the sport," said the Kenyan, who ran the world's unofficial fastest time in Boston in 2011.

The bombs in Boston, which U.S. President Barack Obama has branded an act of terrorism, exploded about four hours into the race and two hours after the men's winner crossed the finish line.

"Normally in a race you have not prepared yourself psychologically that something can happen like that," Mutai said. "So even in your mind you are not free [now] ... it's bringing another thing to sports which is not good."

Despite receiving no assurances about security on Sunday, Mutai has no concerns about his wife watching him bid for a first London title.

"We know that security will be OK because they were organizing the Olympics [last year]," he said.

After a security review with the London force, organizers are promising "considerable extra police" along the 42.2-kilometre (26.2-mile) course.

"One of the great things about the London Marathon is that it is perceived as being this event that brings people together," London Marathon chief executive Nick Bitel said. "One of the founding principles of the London Marathon was to show that at least on one day humanity can be united. We are taking every reasonable step to ensure the race is as safe and secure as possible."

There will be a moment of silence for 30 seconds before the elite men's race and mass start, and runners are being asked to wear black ribbons.

"It's a sign of connection with the people who died over there or got injured," said Patrick Makau, who holds the marathon world record.

Wilson Kipsang, the reigning London champion, said he is angry that Monday's attack appeared to be a deliberate attempt to target athletes.

"There are so many people who came from all over the world to cheer, have fun, compete for charities," he said. "It's even hard to imagine now people would have such an idea in their mind and do something of that.

"But we should have no fear during the race [on Sunday] because security matters will be put in place."