Sadiq Khan says he 'never dreamt' he could become Mayor of London at inauguration at Southwark Cathedral

Sir Ian McKellen, Ed Miliband and Doreen Lawrence were among those watching him be sworn into office

Sadiq Khan has said he “never dreamt” he would become Mayor of London as he was formally sworn in as Boris Johnson’s successor.

Doreen Lawrence, the mother of murdered teenager of Stephen Lawrence, said she had not expected to see an ethnic minority politician during her lifetime at his official inauguration.

Sir Ian McKellen and Ed Miliband were among hundreds of guests present for the ceremony at Southwark Cathedral on Saturday morning, after which Mr Khan announced he would be standing down as an MP.

BSadiq Khan during his swearing-in ceremony at Southwark Cathedral in central London on May 7, 2016. (AFP/Getty Images)

The triumphant Labour politician received a standing ovation as he entered the packed building, which is the seat of the Anglican Church in south London.

“My name is Sadiq Khan and I’m the Mayor of London,” he announced to huge cheers and applause.

“We’re here in Southwark Cathedral because I want to start my mayoralty as I intend to go on.

Sadiq Khan is greeted by Ian McKellen as he arrives for his swearing-in ceremony at Southwark Cathedral in London on May 7, 2016. (AFP/Getty Images)

“I am determined to lead the most transparent, engaged and accessible administration London has ever seen and to represent every single community and every single part of our city as a mayor for all Londoners.

“So I wanted to do this signing in ceremony here, in the very heart of our city, surrounded by Londoners of all backgrounds.”

The democratic presidential hopeful, Hillary Clinton, has offered her congratulations to Mr Khan, writing on Twitter; “Son of a Pakistani bus driver, champion of workers & human rights, and now Mayor of London. Congrats.”

Mr Khan, who was elected on Friday with more than 1.3 million votes, emphasised his message of “hope over fear” amid controversy over his Conservative rival’s campaign.

Zac Goldsmith was accused of attempting to smear his opponent by linking him with alleged Islamist extremists with whom he had attended platforms or represented during his work as a lawyer.

Karen Buck, Ed Miliband, Sir Bernard Hogan-Howe, and London Fire Commissioner Ron Dobson, at Sadiq Khan's swearing in ceremony at Southwark Cathedral in London on May 7, 2016. (AFP/Getty Images)

Mr Khan’s success was seen as an embarrassment for the Tory camp, prompting some Conservatives and Mr Goldsmith’s own sister, Jemima Khan, to criticise their election tactics.

“I can’t quite believe the last 24 hours,” the new Mayor of London said after signing the document officially declaring him mayor, watched by leaders of different faiths.

“I never dreamt I could be standing here…I promise you that I will always do everything in my power to make our city better.”

He had been introduced by Baroness Lawrence, the mother of murdered teenager Stephen Lawrence.

“This really is a glorious day,” she said. ”I never imagined in my lifetime I could have a mayor of London from an ethnic minority.“

Mr Miliband, the former Labour leader, was sitting in the front row alongside Metropolitan Police commissioner Sir Bernard Hogan-Howe.

The Dean of Southwark, Andrew Nunn, told the congregation Mr Khan's victory brought a ”carnival atmosphere“ to the sacred building.

Jeremy Corbyn was conspicuously absent from the ceremony but led congratulations on Twitter, telling the new mayor: ”Can't wait to work with you to create a London that is fair for all“.

Thursday’s election results gave Mr Khan the largest personal mandate of any politician in UK history, generating excitement over his future potential among supporters.

David Lammy, the Labour MP for Tottenham, said that becoming the first Muslim mayor of any EU capital could pave the way for an ethnic minority Prime Minister.

”If we ever get a prime minister of colour, it will be because of what Sadiq Khan has achieved,” he said.

In a Facebook post, the defeated Mr Goldsmith also congratulated Mr Khan and thanked “the hundreds of thousands of people who trusted me with their votes”.

Around 57 per cent of voters supported for Mr Khan, after second preferences were counted, amounting to a landslide victory on the largest turnout in the history of directly-elected mayors in London.

Additional reporting by PA