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How to protect 500,000 along a 26-mile route? London beefs up marathon security

LONDON -- British authorities ordered more police on the streets for Sunday's London Marathon in the wake of the Boston bombings, but experts warned it was "virtually impossible" to guarantee the safety of the hundreds of thousands who will attend the event. 

A police source said additional patrols by uniformed officers were planned to reassure the public in the wake of deadly attack.

While British security officials have been in contact with their counterparts in the U.S. following Monday's blasts, the U.K.'s threat level for international terrorism hasn't been changed from "substantial" -- the third of five categories on the scale.

At least 500,000 spectators are expected to watch Sunday’s race and Prince Harry is due to hand medals to the winners.

The course takes the 36,000 runners right past major sites - including Tower Bridge, the Houses of Parliament and Buckingham Palace – as well as through Canary Wharf, the giant riverside financial district targeted twice by the Irish militants in the 1990s.

Even in a city that has spent recent decades under the threat of bombs – first from Irish Republicans, more recently jihadists – such a public event poses a security headache.

Sir Bernard Hogan-Howe, the commissioner of London's Metropolitan Police, said that the force was "taking more more precautions than we might have done otherwise."

"We will make sure we've got more officers on the street looking after people, making sure they're kept safe, but we've no reason to think they'd be any less safe than before the terrible events in Boston,." he said. "We'd be professionally irresponsible if we didn't take some reasonable steps."

Metropolitan Police Commander Christine Jones declined to give details of what changes might be made, if any, to the event's security plan. She said officers would “continue to review all the intelligence” available.

London Marathon chief executive Nick Bitel insisted the event would go ahead. “We will be reviewing our security in the coming days, in the light of what has happened in Boston," Bitel told ITV News.

"I don't want to talk about specifics of what security we have had in the past, or will have on Sunday. All I can say is that it will be of an appropriate level to meet whatever threat assessment is made, in conjunction with the police," he added.

Hugh Robertson, a British government minister, called for crowds and runners to attend in London as normal.

“The very best way to show solidarity with Boston is to get out there on the streets of London to cheer the runners on and to show that we won’t be defeated by this sort of activity,” he told the London Evening Standard newspaper.

Runners will be encouraged to wear a black ribbon at the start of the race to honor victims of the Boston bombing, and a 30-second silence will be observed, organizers said Wednesday

NBC News national security analyst Michael Leiter said it was “virtually impossible” to make a marathon completely secure because of its 26.2-mile long route.

“You just have to do the best you can to keep people safe and maintain resilience," he said. “It’s important we don’t alter our lives because that provides the terrorist – domestic, international, whoever it may be – with a huge victory.”

Helmut Spahn, executive director of the International Centre for Sport Security, told Reuters: "There has to be a clear analysis of the situation and certainly no over-reaction. More police, more military is not always the best solution. To have a 100 percent security is very, very difficult if not near impossible.”

The German port city of Hamburg is also hosting a marathon Sunday. More than 400 police officers will be on duty.

Organizer Frank Thaleiser said about 22,000 athletes were registered for the event.

"It is impossible to fully control the entire 42 kilometers along the running course, but we have also advised our 3,000 helpers to be extra vigilant and to watch out for abandoned bags or suspicious packages," he said.

"But it does not make sense to position 100 police officers at the finish line, that would only generate panic," he added.

Professor Richard English, director of  the Handa Centre for the Study of Terrorism and Political Violence at Britain's University of St. Andrews, urged people to not be rattled by the Boston attack.

"The chances of people being killed or injured by terrorism are statistically very slight, despite the appalling nature of what happened [on Monday] in Boston," he said. "Continuing normal life makes sense ... In the absence of a well-grounded threat to specific races, the likelihood is that marathons, and most other public occasions, will continue to be safe in the U.S."

NBC News' Ian Johnston contributed to this report.


Full coverage of the Boston Marathon bombings from NBC News