Organizers of world sporting events on alert

Determined to show solidarity with Boston, the London Marathon will go forward as planned Sunday with a race that will be watched by about 1 million spectators and take runners past some of the city's most revered landmarks.

But the deadly bomb attacks on the Boston Marathon raised new concerns over safety at major sports events around the world. It brought into sharp focus the security challenges facing next year's Winter Olympics in Sochi, Russia, and two coming events in Brazil -- the 2014 World Cup and the 2016 Summer Olympics.

"We are very, very concerned," senior International Olympic Committee member Gerhard Heiberg of Norway told The Associated Press. "Security is priority No. 1, no question about it."

Alexander Konovalov, head of the Institute of Strategic Assessment and Analysis, an independent Russian think-tank, said international terror groups could be encouraged by the carnage in Boston to plot against the Sochi Olympics.

"The terrorists' strategy is to create a sense of panic and leave an impression that they can strike any target, no matter how tightly it's protected," Konovalov said.

London, which hosted last year's Summer Olympics without incident, has long been a target for terrorists, and concerns have only intensified after Monday's harrowing scenes at the Boston Marathon.

After high-level talks with police and authorities, organizers Tuesday confirmed that the London Marathon will proceed while acknowledging that a race of more than 26 miles that traverses a city cannot be entirely without risk.

"One of the great things about these marathons is that they are free and available to the public -- that's why we have hundreds of thousands of people come out and watch them," said Nick Bitel, chief executive of the London Marathon. "What one has to do is make appropriate and reasonable security measures in light of the threats."

There will be 30 seconds of silence before the start of the elite men's race and mass start to "mark the tragic events that have unfolded in Boston," organizers said in a statement.