Members of the emergency services who have responded to recent terror attacks and the Grenfell fire disaster are taking part in this year's Pride in London parade.
They are among the thousands making their way through the city as the march set off on Saturday afternoon.
Staff from the Metropolitan Police, London Ambulance Service and London Fire Brigade were nominated to launch the parade after helping in response to recent tragic events in the capital, organisers said.
They joined flag bearers representing countries around the world, including those where it is still illegal to be LGBT Plus.
More than 26,000 people are estimated to be taking part in the parade, which began north of Oxford Circus on Regent Street, watched by a crowd expected to number around one million.
The parade, taking a 1.4 mile (2.3km) route through the city, comes after what is believed to be the world's largest Pride festival, with over 100 events having taken place since Saturday June 24.
The large-scale event is taking place as the country's threat level remains at severe, with police warning people to be vigilant as they enjoy the celebrations.
The Met said it is involved in high visibility police patrols, including both armed and unarmed officers, as well as plain clothes officers.
The parade has been praised by Theresa May, who said London is sending out a "proud and positive message to the world".
The march is also marking 50 years since the Sexual Offences Act 1967, which decriminalised homosexuality in England and Wales.
In a message released ahead of the parade Mrs May, who is currently in Germany at the G20, reiterated the UK's pledge to encourage other countries to ensure equal rights for all, regardless of sexual orientation, and to take a stand against hate crime at home.
She said: "Around the world, cruel and discriminatory laws still exist - some of them directly based on the very laws which were repealed in this country 50 years ago.
"So the UK has a responsibility to stand up for our values and to promote the rights of LGBT Plus people internationally.
"That's why we will continue to stand up for human rights, directly challenging at the highest political levels governments that criminalise homosexuality or practice violence and discrimination against LGBT Plus people.
"And here at home too, we must continue to stand up for true equality and respect for everyone, right across our United Kingdom.
"We must stamp out homophobic bullying in schools, and drive down homophobic and transphobic hate crime."
As part of the annual Pride weekend, a rainbow flag will be projected onto the Palace of Westminster for the first time.
Lord Fowler, Speaker of the House of Lords, said the move will demonstrate the UK's support for those living in countries around the world where people are persecuted for being gay.