Sadiq Khan revealed that he had a "sleepless" night while he waited to hear news about the explosion that rocked New York last night.
The Mayor, in the US city on an official visit, said his "thoughts and prayers" went to those injured by the blast.
After taking advice from the New York authorities he said it was "business as usual" and was pressing ahead with visits to a church, the 9/11 memorial and a Met's baseball game.
Mr Khan, who is expected to meet New York mayor Bill de Blasio later today at an event with Muslim community leaders, said attacks were "part and parcel" of life in a big city.
"It is a reality I'm afraid that London, New York, other major cities around the world have got to be prepared for these sorts of things," he said.
"That means being vigilant, having a police force that is in touch with communities, it means the security services being ready, but also it means exchanging ideas and best practice."
He added: "I've had a sleepless night thinking about all of this and I'm sure the Mayor has as well."
Mr Khan criticised US presidential candidate Donald Trump - who said within minutes of the attack it was time to "get tough" - for speculating on the motives behind it.
"I'm not going to speculate as to who is responsible or how the police in New York should react. Speculating when you don't know the facts is unwise," he told The Standard.
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