A road close warning sign is seen in central London, capital of Britain, on April 20, 2013. London's Metropolitan Police have reinforced security for the competition scheduled for April 21, in the wake of the Boston Marathon bombings. (Xinhua/Gautam) 

Security personnel work at the Mall, the finishing line of the London Marathon, in London, capital of Britain, on April 20, 2013. London's Metropolitan Police have reinforced security for the competition scheduled for April 21, in the wake of the Boston Marathon bombings. (Xinhua/Gautam)

A security man works with an explosive-detecting dog at the Mall, the finishing line of the London Marathon, in London, capital of Britain, on April 20, 2013. London's Metropolitan Police have reinforced security for the competition scheduled for April 21, in the wake of the Boston Marathon bombings. (Xinhua/Gautam)

A security man works with an explosive-detecting dog at the Mall, the finishing line of the London Marathon, in London, capital of Britain, on April 20, 2013. London's Metropolitan Police have reinforced security for the competition scheduled for April 21, in the wake of the Boston Marathon bombings. (Xinhua/Gautam)