Sadiq Khan welcomed the mayor of Paris Anne Hidalgo to London, after she jumped on the Eurostar to congratulate him on his mayoral election victory.
Khan met Hildalgo at St Pancras station where the two toasted their relationship with a glass of water as they had a meeting in a cafe.
Speaking to French and British reporters on a cramped platform at St Pancras International, Khan said: "Can I say it's an honour to have Anne here in London.
"What's really important is the similarities Paris and London have - they are the most diverse cities in the world.
"This is an example of the best of our cities - men, women, Muslims, Christians, mayors working together to work for our cities.
"Solving the housing crisis, fixing the air quality, addressing the challenges of integration and making sure our cities are safe.
"Our message to Donald Trump is this is how you work together. This is the best of humanity, this is the best of the West."
He added: "You can be a Parisian mayor and a London mayor and work closely together.
"I hope Donald Trump looks at the lessons that London sent last Thursday and recognises that it's possible to be Western and Muslim and to be friends with a mayor of Paris as well."
Hidalgo was scathing about the Republican presidential hopeful's policy of banning all Muslims from the US - but making an exception for Khan.
She said: "Donald Trump is stupid, he is very stupid."
She said people of all religions, including Catholics and Muslims, do not agree with Trump, adding: "Mr Trump is so stupid, my God, my God."