Newly-elected London mayor Sadiq Khan has been formally signed in to the role at a Southwark Cathedral ceremony.
Mr Khan said: "I'm determined to lead the most transparent, engaged and accessible administration... and to represent every single community, and every single part of our city, as mayor for all Londoners."
Former Labour leader Ed Miliband and Baroness Doreen Lawrence were present.
Mr Khan received a standing ovation as he walked in to the hall.
Dean of Southwark Andrew Nunn told the congregation the new mayor's victory brought a "carnival atmosphere" to the sacred building.
Mr Khan has said he will stand down as Labour MP for Tooting.
Earlier, Labour MP for Tottenham, David Lammy, said the first Muslim mayor of any capital city in the EU could pave the way for a "prime minister of colour".
Having won 1,310,143 votes, Mr Khan received the largest personal mandate of any politician in UK history.
The son of a London bus driver who grew up in the capital, he is the third person to become the mayor of London after fellow Labour politician Ken Livingstone and Conservative Boris Johnson.
At the ceremony, Mr Khan said he was "truly humbled".
Signing in as the mayor of London, he was greeted with cheers as he said: "My name is Sadiq Khan and I'm the mayor of London."
He evoked laughter by saying: "Some of you may not know this, but I grew up on a council estate, just a few miles from here.
"Back then, I never dreamt that I could be standing here as the mayor of London.
"I'm only here today because of the opportunities and helping hand that our city gave to me and my family.
"And my burning ambition for our city, that will guide my mayoralty, is to ensure that all Londoners get the opportunities that our city gave to me.
"I want to start my mayoralty as I intend to go on. I'm 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 mayor for all Londoners."
Campaigner Baroness Lawrence, mother of murdered black teenager Stephen, said: "I never imagined in my lifetime I could have a mayor of London from an ethnic minority."
Earlier, describing the human-rights lawyer, Mr Lammy said: "If we ever get a prime minister of colour it will be because of what Sadiq Khan has achieved."
Mr Lammy, who stood against Mr Khan for the Labour mayoral nomination, told the BBC Radio 4 Today programme Mr Khan "is a grafter, he is someone who gets on with people, he is someone who is pragmatic when he needs to be and he certainly has a vision for this city".
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said: "I look forward to working with him, and all London's new and re-elected Assembly members, to continue transforming the Met to keep London one of the safest capitals in the world."
Defence Secretary Michael Fallon, who during the mayoral campaign had branded Mr Khan a "Labour lackey who speaks alongside extremists", defended the Tory tactics as the "rough and tumble" of the campaign.
On Today programme he said: "London is safe with a Conservative Government working with the new Mayor of London."
Following Mr Khan's victory on Friday night, Labour party leader Jeremy Corbyn tweeted the hashtag #YesWeKhan, telling the new mayor: "Can't wait to work with you to create a London that is fair for all".
Outgoing mayor Boris Johnson said: "Many congratulations to Sadiq on securing a huge mandate to do the best job in British politics. I wish him every possible success."
Conservative candidate Zac Goldsmith also wished the new mayor "well".
Praise also came in from New York mayor Bill de Blasio and Business Secretary Sajid Javid, who tweeted: "From one son of a Pakistani bus driver to another, congratulations".