The Mayor of New York during the 9/11 attacks, Rudolph Giuliani, has spoken of the "eerie" experience of being in London near one of Thursday's blasts.
Mr Giuliani said people must stand up to terrorism
He told the BBC he was near Liverpool Street station when the first of the four blasts was reported.
Mr Giuliani described the explosions as "dastardly, cowardly acts".
"The people of London should know that they have, in the people of New York, people who understand what they are going through," he said.
"My heart goes out to the people affected by this, it reminds me so much of September 11," he said.
Mr Giuliani was praised for taking to the streets to reassure New Yorkers immediately after the 9/11 attacks.
His opposite number, London Mayor Ken Livingstone, is on his way back from Singapore where he was part of the team which secured the 2012 Olympics.
Mr Giuliani said no nation had offered the US more support after the 9/11 attacks than Britain - and now that Britain had been attacked, it was the turn of the US to offer its ally support.
"I was right near Liverpool (Street) station when the first bomb went off and was notified of it," he said.
"It was... very eerie to be right there again when one of these attacks takes place."
Mr Giuliani, whose eight-year tenure as mayor of New York will be best remembered for his heart-felt response to the attacks on the World Trade Centre, drew parallels between events on both sides of the Atlantic.
He said "the idea of a surprise attack", as well as "innocent people just going to work being injured and killed", meant the attacks bore similarities.
He said New Yorkers recovered from the September 11 attacks and predicted that Londoners would do the same.
The former mayor said the best way to react to such attacks is "to stand up to them and do everything we can to support the people that were affected by it - but not to let these terrorists affect our way of life".
He went on: "People in London have always been remarkable. They were our inspiration on September 11 when we thought about how they reacted to the blitz in 1940, and how they went through something far worse than we would ever have to go through.
"It isn't surprising that their children, grandchildren and great-grandchildren have that same resolute response to this attack."