Image caption The explosions happened at what is one of the biggest sporting events in the US

There will be a 30-second silence at the start of the London Marathon on Sunday, and runners will be given a black ribbon to wear, organisers say.

Race director Hugh Brasher said organisers wanted to show support for Boston, after the bombings which killed three and injured more than 150.

The UK Foreign Office said it was following up reports that a British national was among the injured.

Met Police chiefs sought to reassure people the London race would be safe.

The Foreign Office was unable to give details of the British national, but did say embassy staff were following up reports of an injury and "stand ready" to offer consulate assistance.

Of the London Marathon, Met Police Commissioner Sir Bernard Hogan-Howe said: "We are reviewing our plans which is sensible... We will increase searching, we will make sure we've got more officers on the street looking after people, making sure they're safe."

He added: "We've no reason to think they're any less safe than they were before the terrible events in Boston yesterday."

Event organisers said the race's runner registration and associated events would begin on Wednesday as planned.

The silent tribute will be held at the start of the elite men's race and mass start.

'Enormous experience'

London Marathon chief executive Nick Bitel earlier said: "The support we have been offered by our stakeholders and the wider running community has been outstanding. We have the full support of the Metropolitan Police, the Mayor's office and other authorities.

"We want to reassure our runners, spectators, volunteers and everyone connected with the event, that we are doing everything to ensure their safety and that the Virgin London Marathon 2013 is an outstanding success."

London Mayor Boris Johnson has also spoken to police about the possibility of extra security on the day.

"We do have robust security measures in place, but given events in Boston it's only prudent for the police and the organisers of Sunday's race to re-examine those security arrangements," he said.

Sports Minister Hugh Robertson said he was "absolutely confident" the London Marathon could be kept safe.

London had "enormous experience" of delivering major events and the UK had some of the best security professionals in the world, he told the BBC, adding: "This is one of those instances where the best way to show solidarity with Boston is to continue."

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

Image caption The London Marathon takes place this Sunday with about 36,000 people due on the starting line

St James's Palace said Prince Harry would still attend the marathon to make presentations to the winners.

The London Marathon route, which is lined by hundreds of thousands of spectators every year, starts in Blackheath and finishes near Buckingham Palace, passing some of the capital's most recognisable landmarks including Tower Bridge, Canary Wharf and Big Ben.

Paula Radcliffe, three times winner of the London marathon, urged people attending the race to be vigilant.

"I have every confidence in the London Marathon community and the Metropolitan Police that they won't let it go ahead unless they have done everything possible," she said in a BBC interview.

"But it's still 26 miles...it's going to be hard. It's going to rely on people being vigilant and aware of what is going on around them as well.

"I think first and foremost as a mother I'd think more about having family at the finish area."

'Total disbelief'

In Boston, the first of the explosions went off close to the marathon finish line at about 14:50 local time (18:50 GMT) on Monday.

Seconds later, as rescuers were rushing to help the injured, another explosion went off nearby.

Among the dead were an eight-year-old boy, Martin Richard, from Boston. Doctors treating the wounded have said a number of people have had pellets and "nail-like" fragments removed from their bodies, and there are reports of several amputations.

According to the Boston Marathon website, 374 of the more than 25,000 registered entrants were British - and Britons caught up in the incident have been describing their experiences.

Marathon runner Lynn Burnet, from Nairn, in the Highlands, told how she sprinted to find her two children after hearing the explosions.

"I heard this awful sound. It was boom, boom. People started to look panicked. I forgot I had run a marathon running to find them," she said.

Ms Burnet then carried one daughter for a mile as they fled to safety.

Former boxing world champion Eamon Loughran, from Ballymena, County Antrim, has described how he prayed as he waited to find out if his wife was safe after the explosions.

He had arranged to meet her at the finishing line but faced a three-hour wait before he was reunited with her.

"I was crying, I was thinking the worst because that's where I told her to be," he said.

Another runner, Owain Griffiths, from Cyncoed in Cardiff, said "a day of enjoyment" had descended into "unthinkable horror".

Police say they have also launched a review of security for the Greater Manchester Marathon, due to take place on Sunday 28 April.