Police patrolling the London marathon will be on high alert, with numbers boosted in the wake of the Boston Marathon bombings, the Home Secretary has told MPs.
Theresa May said adjustments to security arrangements had been made to this weekend's race after the fatal explosions that tore through spectators watching the race in the United States on Monday.
A policeman watches last year's London Marathon
Giving evidence to the Home Affairs Select Committee on Thursday, May said: "They've made some adjustments to policing. They've put in some increased policing for the marathon."
During the Boston Marathon, held on Patriots' Day, two pressure cooker bombs exploded, killing 3 people and injuring 183 others.
No-one has been arrested or claimed responsibility for the bombing, which is being investigated by the FBI.
This weekend's London marathon will be preceded by 30 seconds silence for the victims of the Boston attack, and runners are being in encourage to put their hands on their hearts as they cross the finish line.
In his weekly LBC radio phone-in, Nick Clegg said he understood the anxiety of runners and their supporters, but had confidence in the security arrangements.
The Deputy Prime Minister said: "I have spoken to the Met Commissioner himself about this yesterday, and I have a huge amount of confidence in the police and security services of this country.
"They do an amazing job keeping us safe all the time. They are doing a double, triple, quadruple checking of all their arrangements for the Marathon, because of course what happened in Boston is just horrific.'"
Marathon chief executive Nick Bitel said the organisers had "instigated a full security review. Considerable extra police and our own security resources will be employed.
"London is one of the safest cities in the world. We saw last year with the Olympic Games the Metropolitan Police lead an incredible security effort around the Games. There is really no finer force in the world in answering these type of threats."