Extra police and security staff will be stationed along the route of the London Marathon tomorrow as one of many extra measures being taken by race organisers after consultations with the Metropolitan Police and the Mayor's office.
Increased screening at the registration points for the 37,500 runners has also been introduced after the bombings that killed three people at the Boston Marathon on Tuesday, says London Marathon chief executive Nick Bitel.
The Home Secretary, Theresa May, told MPs more police would be at the Marathon and adjustments to security arrangements had been made.
The Deputy Prime Minister Nick Clegg said everything possible was being done to ensure the marathon passed safely.
In his weekly radio phone-in, Clegg said police had been "double, triple, quadruple checking" arrangements for this weekend's event.
"I have spoken to the Met Commissioner [Sir Bernard Hogan-Howe] about this yesterday, and I have a huge amount of confidence in the police and security services of this country," he said.
"They do an amazing job keeping us safe all the time."
Bitel was reluctant to reveal any further details but he has been assured by London's Mayor, Boris Johnson, that budgetary concerns will have no bearing on any extra measures deemed necessary.
Bitel will also have been buoyed by the support of several top runners competing tomorrow. Defending champion Wilson Kipsang said he believed they would run with "no fear".
"One of the founding principles of the London Marathon was to show that at least on one day humanity can be united," said Bitel.
"We are taking every reasonable step to ensure the race is as safe as possible. Considerable extra police and our own security resources will be employed. The Mayor made it clear to the commissioner [of the Metropolitan Police] that it is about putting out the right number to send the right message. It is not about money, it is about ensuring the police are able to do as they do every day and keep London safe and secure."
Kipsang, from Kenya, said: "I would like to send our condolences to those people who lost loved ones in Boston. We know they are going through a hard time," he said. "We are sorry for what happened but we should have no fear during the race because security matters will be put in place. We are going to run feeling free because something of that kind rarely happens."
Former Boston Marathon winner Geoffrey Mutai called for security to be stepped up at the finishing line.
"They have taken our freedom which we normally have in races," the Kenyan said. "When you are in a race you are relaxed and you are enjoying yourself. But now there must be watertight security."