His expected victory may be a lone bright spot for Britain's main opposition party in local elections in England, Scotland and Wales. Opinion polls and voters suggested Labour would lose seats in some traditional strongholds, testing the authority of its new left-wing leader, Jeremy Corbyn.
Britons trickled in to voting stations to cast their ballots in elections that some campaigners fear could fail to attract many voters, as the contests have been overshadowed by next month's referendum on whether Britain should leave the European Union.
The fight to run London - the top prize in the local elections - has pitted Labour's Khan, 45, the son of an immigrant bus driver, against Conservative Zac Goldsmith, 41, the elite-educated son of a billionaire financier.
The winner will replace Conservative Boris Johnson, who has run the city of 8.6 million people for eight years. A top campaigner for Britain to leave the EU, Johnson is seen as a contender to succeed David Cameron as party leader and prime minister.