Digitally generated audio of Sadiq Khan seemingly calling for Armistice Day to be delayed is being investigated by police.
Clips have been circulating on social media, using the London mayor's voice and mannerisms, where he can be heard playing down the importance of Remembrance weekend commemorations.
In one clip, a voice similar to Mr Khan can be heard saying: "I don't give a flying s*** about the Remembrance weekend."
The fake recording continues to say: "What's important and paramount is the one million-man Palestinian march takes place on Saturday."
It's a reference to the Million March in 1965 - a civil rights protest in Washington DC attended overwhelmingly by people of colour.
A large pro-Palestinian demonstration in London calling for a ceasefire in Gaza has been planned for Saturday, with more than 2,000 police officers drafted in to help manage the event.
However, Home Secretary Suella Braverman has criticised the Metropolitan Police over its decision to allow the march go ahead.
Another clip using Mr Khan's voice says: "I know we have Armistice Day on Saturday but why should Londoners cancel the Palestinian march on Saturday? Why don't they have Remembrance weekend next weekend? What's happening in Gaza is much bigger than this weekend and it's current."
The Metropolitan Police said it was investigating the fake clips.
"We can confirm that we have been made aware of a video featuring artificial audio of the mayor, and that this is with specialist officers for assessment," the force said in a statement.
Writing on social media, Mr Khan wrote: "While I hosted an interfaith Remembrance event with our armed forces at City Hall: the far-right were sharing deepfake audio about me.
"They may have new means, but their ends are the same - to divide our diverse communities. We must stand together - it's what London does best."
The mayor also used social media to point people to an article written in the Evening Standard about the importance of events this weekend.
In it, he writes: "It's right that the organisers have said they will not protest near the Cenotaph. I urge everyone attending to co-operate with police and make sure to be respectful on Armistice Day."
People's comments on the faked audio ask if it is real or made with artificial intelligence - an indication of how accurate the technology used to make these kind of clips is.
It comes after a deepfake clip of Mr Khan's party leader, Sir Keir Starmer, was circulated as Labour's annual conference got under way in October, heightening fears about the potential impact of the technology on democracy.