After a manhunt which left Boston in virtual lockdown, 19-year-old Dzhokhar Tsarnaev was arrested following a shootout with officers and his brother Tamerlan, 26, was killed yesterday.

But several hundred more officers will still be out on London’s streets tomorrow - a 40% increase on last year.

The Met is in close contact with the FBI and Boston Police and there is nothing to suggest any link with the bomb attacks.

A 30-second silence will be held as a mark of respect before the start of the men’s elite race and mass start, and runners are being encouraged to wear black ribbons.

Virgin London Marathon has also pledged to donate £2 for every finisher in Sunday’s event to The One Fund Boston set up to raise money for victims of the explosions.

The race’s chief executive insisted the bloody events in Massachusetts will not put off spectators or runners taking part in the 26.2 mile event.

Nick Bitel said the boosted police presence would help ensure the event will be “as safe and fun as it always is”.

He told BBC Radio 4’s Today programme: “After the terrible events in Boston we conducted a security review with the Metropolitan police and all the other agencies and, as was announced yesterday, we have increased substantially the number of police officers who are going to be out there, going to be visibly reassuring people and putting in more search regimes and a number of other measures to ensure that it is as safe and fun as it always is.”

This was an “appropriate response level” but “we don’t want to lose the feeling that is such an important part of the Virgin London Marathon and I think people will come out and cheer and have a great day”.

He added: “I think the spectators won’t be put off, we’ll have a great day. There’s some fantastic races, really competitive, and the numbers in terms of people running will be 35,500 – 36,000 as they would always be.”

And with blisters, fatigue, aches and pains to contend with, those taking part are unlikely to notice the extra few hundred officers.

Events in Boston reached a dramatic climax as Dzhokhar Tsarnaev was apprehended following an exchange of gunfire with police after being found hiding in a boat in a back yard in the Boston suburb of Watertown.

He was taken away on a stretcher and was admitted to hospital in a serious condition with unspecified injuries.

News of his capture led to jubilant celebrations at the scene and across the city.

In a televised address from the White House, US president Barack Obama said the arrest of the 19-year-old “closed an important chapter in this tragedy”.

But he acknowledged that there were many unanswered questions about the motivations of the two men accused of perpetrating the attacks that unnerved the nation.

“The families of those killed so senselessly deserve answers,” said Mr Obama, who branded the suspects “terrorists”.

The brothers – ethnic Chechens who had lived in the Dagestan region in southern Russia before moving to the US around 10 years ago – are also suspected of killing a Massachusetts Institute of Technology police officer in his vehicle on Thursday.

The authorities in Boston had suspended all mass transit and warned close to a million people in the city and some of its suburbs to stay indoors as the hunt went on.