Sadiq Khan of Britain's opposition Labour Party was sworn in as Mayor of London on Saturday, becoming the first Muslim to head a major European capital after an election campaign marked by the ruling Conservatives' efforts to link him to extremism.

The son of a Pakistani immigrant bus driver and a seamstress, Khan defeated Conservative candidate Zac Goldsmith, the son of a billionaire financier, by a record margin to secure the biggest individual mandate in British political history.

The Financial Times said the result highlighted London's tolerance, a "remarkable triumph over the racial and religious tensions that have bedevilled other European capitals".

"My name is Sadiq Khan and I'm the Mayor of London," the 45-year-old said to wild applause at the ceremony at Southwark Cathedral attended by the city's police chief, politicians and leaders of different faiths.

Khan's election also puts a supporter of Britain remaining in the European Union at the helm of the global financial centre, even though the issue barely came up in the campaign.

Goldsmith and outgoing mayor Boris Johnson favour a vote to leave when Britain holds a referendum on the issue next month.

Politicians from all sides lined up to condemn the Conservative Party tactics in the race, but in the aftermath, Defence Secretary Michael Fallon refused to apologise.

"In the rough and tumble of elections, you get stuff said, questions asked," Fallon told the BBC. "I think it is right that candidates for some of the most important offices in Britain do get scrutinised about their past associations."

Conservatives including Prime Minister David Cameron and Fallon himself had questioned whether London would be safe under the control of Khan, a former human rights lawyer who grew up in public housing in the capital's inner city.

"They used fear and innuendo to try to turn different ethnic and religious groups against each other – something straight out of the Donald Trump playbook," Khan told the Observer newspaper.

Many commentators said the focus on religion had backfired in a city noted for its diversity.

During the race Goldsmith had joined forces with Cameron and other senior party members to question Khan's past appearances alongside radical Muslim speakers at public events, accusing him of giving "oxygen" to extremists.

During one heated session in parliament, Labour lawmakers accused Cameron of racism when he repeatedly raised the issue.

CONTROVERSY, CONGRATULATIONS

Khan said he had fought extremism all his life and regretted sharing a stage with speakers who held "abhorrent" views.

Sayeeda Warsi, a former Conservative Party chairman, said the campaign had damaged the party's credibility on issues of race and religion, while Labour politicians called on Fallon and Cameron to stop smearing their candidate.

The left-wing mayors of New York and Paris saluted Khan, as did U.S. presidential candidate Hillary Clinton.

"Son of a Pakistani bus driver, champion of workers' rights and human rights, and now Mayor of London. Congrats, @SadiqKhan," Clinton said on Twitter.

Sajid Javid, Britain's business secretary and a Conservative, said on Twitter: "From one son of a Pakistani bus driver to another, congratulations."

Khan's victory makes him the first Muslim to head a major Western capital. During the campaign he had vowed to tackle the lack of affordable housing in London, oppose the expansion of Heathrow Airport and improve the transport system.

The victory in London was one of the few bright spots for Labour, which endured poor results in elections elsewhere, especially in Scotland. However leader Jeremy Corbyn did not attend Khan's swearing-in ceremony.