New London mayor Sadiq Khan won a landslide victory Saturday, becoming the first Muslim leader of a Western capital, boosting the Labour party and raising questions over failed Conservative tactics.

Khan, whose Pakistani parents emigrated to Britain in the 1960s, beat Conservative multimillionaire Zac Goldsmith with 57 percent or 1.3 million votes – handing him the largest mandate of any British politician in history.

"Thank you London. London is the greatest city in the world. I am so proud of our city. I am deeply humbled by the hope and trust you have placed in me today," Khan said in his acceptance speech in the early hours.

"I grew up on a council estate... back then I never dreamt that someone like me could be elected as mayor of London.

"I want every single Londoner to get the opportunities that our city gave to me and my family – the opportunities not just to survive, but to thrive."

Khan swept to victory despite repeated insinuations over extremist links from his rivals, including Prime Minister David Cameron.

Khan repeatedly rejected the allegations and Labour accused Goldsmith of Islamophobia.

"This election was not without controversy and I am so proud that London has today chosen hope over fear and unity over division," the 45-year-old added.

"I hope that we will never be offered such a stark choice again. Fear doesn't make us safer, it only makes us weaker, and the politics of fear is simply not welcome in our city."

The victory offered some cheer for Labour leader Jeremy Corbyn after setbacks elsewhere following regional elections across Britain on Thursday, amid a damaging row over anti-Semitism in the party.

Khan succeeds the charismatic Boris Johnson after a two-year term.

The London mayoralty has responsibility for transport, housing, policing and promoting economic development, but also carries prestige and political clout at national level.

News of the win was applauded in Pakistan.

"Congratulations @SadiqKhan 4 being elected mayor of London," tweeted Bilawal Bhutto, leader of the opposition Pakistan People's Party and son of former prime minister Benazir Bhutto.

"British Pakistanis need... rolemodel," the tweet added.

Rival opposition leader and former cricketer Imran Khan – whose ex-wife Jemima is the sister of Zac Goldsmith – also tweeted congratulations.

Paris Mayor Anne Hidalgo added on Twitter that Khan's "humanity, progressivism will benefit Londoners".

The victory was hailed in Tooting, a multi-ethnic area of south London where Khan lives and where he grew up in social housing, before becoming first a human rights lawyer and then a member of parliament.


'Appalling dog whistle campaign'

There were however recriminations in the Conservative camp.

Former Conservative Cabinet minister and party co-chair Sayeeda Warsi warned: "Our appalling dog-whistle campaign lost us the election, our reputation and credibility on issues of race and religion".

Goldsmith's sister Jemima added the way the contest was fought "did not reflect who I know him to be".

Cameron's former adviser, Steve Hilton, said Goldsmith had brought back the "nasty party label".

At the same time, Warsi and Conservative business minister Sajid Javid – who like Khan were also children of Pakistani immigrant bus drivers – both sent their congratulations to the new mayor.

Labour meanwhile fared less well in other elections on Britain's "Super Thursday", in which 45 million Britons were eligible to vote.

The party came third in elections for the devolved government in Scotland, behind the Conservatives, in a vote won by the incumbent pro-independence Scottish National Party (SNP).

Labour retained power in the Welsh assembly, although it lost one seat, and with 118 of 124 results declared, maintained all but one of its local councils in England.

Corbyn, a veteran socialist who has faced opposition from centrists in his party since being elected last year, said his party had surpassed expectations.

But critics said Labour should have made more gains given the splits in the Conservative government before the EU referendum on June 23, and its unpopular spending cuts.

SNP maintains power

SNP leader and Scottish First Minister Nicola Sturgeon will lead the separatist party into its third successive government in Edinburgh, although the party lost its outright majority.

She announced she would lead a minority administration, and played down talk of a fresh independence referendum to follow the unsuccessful plebiscite in 2014.

The other big story in Scotland was the success of the Scottish Conservatives, who came second with 31 seats.

The party has been deeply unpopular in Scotland since the 1980s premiership of Margaret Thatcher but its fortunes have turned around under current Scottish leader Ruth Davidson.

Elsewhere, the anti-European, anti-immigration UK Independence Party (UKIP) secured its first seat in the Welsh assembly and two seats in the London assembly.