Khan, the first Muslim mayor of London, said Londoners stood shoulder to shoulder with the people of Christchurch.
He also reassured Muslims and those of other faiths that there would be an increased police presence in the British capital.
At least one gunman killed 49 people and wounded more than 40 during Friday prayers at two New Zealand mosques in the country’s worst ever mass shooting, which Prime Minister Jacinda Ardern condemned as terrorism.
A gunman broadcast livestream footage on Facebook of the attack on one mosque in the city, mirroring the carnage played out in video games, after publishing a “manifesto” in which he denounced immigrants, calling them “invaders”.
Police said three people were in custody including one man in his late 20s who had been charged with murder.
He will appear in court on Saturday (March 16).
Leaders around the world expressed disgust and sorrow, and some also expressed anger at what they described as the demonisation that fuelled such attacks.