Britain's most senior police officer said security around Baroness Thatcher's funeral and the London Marathon will be stepped up in the next few days following the bombings in Boston.
Sir Bernard Hogan-Howe said that more searches will be carried out and there will be a greater number of officers on the streets.
Speaking outside Scotland Yard, he said: "I just want to reassure people that we've got some really good plans in place. We're doing an awful lot of work already to keep London safe and I'm sure that will happen over the next few days. We are reviewing our plans which is sensible, but our reassurance is that we will keep people in London safe.
"We will increase searching, we will make sure we've got more officers on the street looking after people, making sure they're safe. We've no reason to think they're any less safe than they were before the terrible events in Boston yesterday."
He said "no obvious link" has yet been discovered between the atrocities in Boston and the United Kingdom.
"Obviously the Americans are trying their best to discover who committed this heinous crime and where they are. They're doing their best to explain to us what's happening and we're talking to them all the time.
"We're just taking more precautions that we might have done otherwise. It's a very reasonable response to an event such as we've seen.
"We'd be professionally irresponsible if we didn't take some reasonable steps and you would be asking me questions 'what are you doing to look at the London Marathon?'
"It's a very sensible thing to review our precautions. First of all we'll deal with the events tomorrow, the funeral in London of Baroness Thatcher, and make sure that's carried out safely, and then we'll address our concerns towards the London Marathon on Sunday."
"In between times we'll hopefully get more information from the Americans as they try and work out what happened, who committed this crime, and it may be of course that there is no link at all to the United Kingdom, in which case any potential threat against the marathon is defused."