Labour's Sadiq Khan has vowed to do all in his power to make London "better", as he was sworn in as the new mayor.
Referring to his council estate roots, Mr Khan, the city's first Muslim mayor, said he wanted all Londoners to have the same opportunities he has had.
It comes as Defence Secretary Michael Fallon defended Conservative Zac Goldsmith's campaign, describing it as the "rough and tumble" of politics.
The much-criticised campaign questioned Mr Khan's alleged links to extremists.
Mr Khan beat Conservative Zac Goldsmith, by 1,310,143 votes to 994,614 - a margin of victory of 13.6%, giving him a larger personal mandate than either of his predecessors, Boris Johnson and Ken Livingstone.
He has announced he will step down as MP for Tooting, meaning there will be a by-election in the constituency to elect a new representative in Parliament.
Meanwhile, Nicola Sturgeon has introduced the SNP's new batch of MSPs as the party celebrates its third successive victory in the Holyrood election - though the result left the SNP two seats short of a majority in the parliament.
The second day of counting in the Northern Ireland elections is nearing an end, with the Democratic Unionists certain to remain the biggest party in the assembly, and Arlene Foster continuing as first minister.
A handful of English councils are also expected to declare their results.
'Mayor for all'
At the swearing in ceremony in Southwark Cathedral, Mr Khan, the son of Pakistani immigrants who was brought up on a council estate, said that as a child he "never dreamt" he would one day become the mayor of the capital.
"I'm only here today because of the opportunities and helping hand that our city gave to me and my family.
"My burning ambition for our city, that will guide my mayoralty, is to ensure that all Londoners get the opportunities that my city gave to me," he said.
"I promise you I will always do everything in my power to make our city better. I will be a mayor for all Londoners," he added.
He also pledged to lead "the most transparent, engaged and accessible administration London has ever seen".
'Rough and tumble'
During the mayoral campaign, Mr Khan was accused by rival Conservative candidate Mr Goldsmith of sharing a platform with Islamist extremists - a strategy which has drawn criticism from Labour and some Conservatives.
Former Conservative cabinet minister Ken Clarke has said the party's campaign tactics were a "mistake" and "probably had a counter-productive effect", while Baroness Warsi said it had "lost us the election, our reputation and credibility on issues of race and religion".
Mr Goldsmith's sister Jemima also weighed in with criticism of the way the contest was fought, saying it "did not reflect who I know him to be".
Referencing that campaign in his victory speech after the results of the contest were announced, Mr Khan said he was "proud" that voters had "chosen hope over fear".
But Defence Secretary Michael Fallon - who called Mr Khan a "Labour lackey who speaks alongside extremists" during the election campaign - has defended the Conservatives' approach, telling the BBC it was legitimate to put a candidate under scrutiny.
"Both candidates were asked questions about their backgrounds, their personalities, their judgements, the people they associate with. That's the nature of our democracy and the rough and tumble of politics," he said on BBC Radio 4's Today.
Repeatedly challenged over whether he believed Mr Khan was a security risk to London, Mr Fallon said: "London is safe with a Conservative government working with the new mayor of London."
The new mayor did not have a privileged start in life. He was one of eight children born to Pakistani immigrants, a bus driver and a seamstress, on a south London housing estate.
From an early age, he showed a firm resolve to defy the odds in order to win success for himself and the causes important to him.
That resolve has won him the biggest personal mandate in the UK, a job with wide-ranging powers over London and with enormous emotional significance for him.
Some question whether he has the experience or record of good judgement necessary for the role.
He insists he is there to represent all Londoners and to tackle inequality in the capital, and now he has the chance to prove it.
Labour's Tottenham MP David Lammy said Mr Khan had risen above what he described as "smears" and predicted his election could lead to an ethnic minority candidate entering Downing Street.
"If we ever get a prime minister of colour it will be because of what Sadiq Khan has achieved," he told Today.
Meanwhile, John McTernan, a former strategist to Tony Blair, told the BBC Mr Khan's election was a "rejection of hate" and showed Labour can "win a victory".
He said it was good for Labour leader Jeremy Corbyn "in the short term" but noted that Mr Khan's campaign was "totally independent" of Mr Corbyn and the politics he articulated.
"This creates a new powerbase in the Labour party," he added.
Mr Khan distanced himself from Mr Corbyn during the campaign, pledging to freeze fares on the capital's transport network and build more affordable housing, but also promising to champion business and cut taxes on enterprise.
The Labour leader has welcomed Mr Khan's victory in London and commenting on the party's wider performance in Thursday's elections, said: "We hung on and we grew support in a lot of places."
In total, 2,747 seats in English councils - spanning metropolitan boroughs, unitary authorities and district councils - were up for grabs, in the single largest test of political opinion before the next general election, scheduled for 2020.
Labour's vote share is down about 6% on average on 2012 - the last time these seats in England were contested - with 24 fewer councillors. But its share is up 4% on the general election in key wards, with the Conservatives down by a similar amount.
In Scotland, Labour was beaten into third place by the Conservatives - an unthinkable result in the past.
David Cameron said the party's second place in Scotland and its showing in England, where it took control of Peterborough Council and won council seats in key Westminster marginals such as Dudley and Nuneaton, represented a good result for a party which had been in government for six years.
In Wales, Labour's vote is down by eight points overall, the Conservative vote is down by three points, and Plaid Cymru is up by two points. UKIP increased its vote by 12 points and saw seven candidates elected.