The London Marathon will go ahead on Sunday as planned despite the two huge explosions at Monday's Boston marathon .

But security arrangements for the event and for former British prime minister Margaret Thatcher's funeral on Wednesday are being reviewed.

Queen's Life Guards and police officers ride down the Mall in London on Tuesday where the finish area of the forthcoming London Marathon is to be located. Sang Tan / Associated Press

British Sports Minister Hugh Robertson said he is "absolutely confident" the London race, involving about 37,500 competitors, can be kept safe.

London has "enormous experience" in delivering major events, and the UK has some of the best security professionals in the world, he told the BBC just 12 hours after the blasts. He said he was horrified by the Boston blasts. "This is one of those instances where the best way to show solidarity with Boston is to continue," he added.

Home Secretary Theresa May has been briefed on the bombings by Scotland Yard Assistant Commissioner Cressida Dick and MI5 chief Sir Jonathan Evans, according to media reports.

The current threat level to the UK from international terrorism, set by the Joint Terrorism Analysis Centre, remains at substantial. The Metropolitan Police are reviewing security preparations for the London Marathon and for Thatcher's funeral.

More than 4,000 officers will be on duty during Thatcher's funeral in an attempt to guard against violent demonstrations.

In a brief statement, Chief Superintendent Julia Pendry, in charge of policing the London Marathon, said: "A security plan is in place. We will be reviewing our security arrangements in partnership with the London Marathon."

An estimated 500,000 spectators will line the streets to watch Sunday's race.

Zhang Xia, who is in her 30s and lives in London, told China Daily, "I watched the London Marathon last year with my daughter and husband. It's a fantastic and encouraging event, attracting many spectators. "We are considering whether we should watch it again this Sunday, after the blasts in Boston."

London Marathon chief executive Nick Bitel said: "Our security plan is developed jointly with the Metropolitan Police, and we were in contact with them as soon as we heard the news."

London Mayor Boris Johnson said: "The bombings in Boston are shocking, cowardly and horrific, and the thoughts of all Londoners this morning will be with the victims. Boston is a proud city built on history, tradition and a real sense of community."