Mayor of London Sadiq Khan has launched the capital's Pride parade with an attack on would-be prime minister Boris Johnson for using "homophobic language".
Mr Khan said the Conservative Party leadership favourite had used "the same sort of language" as homophobes who have mistreated LGBT people, and called on him to "realise that language matters".
Hundreds of thousands of people took to the streets of central London to celebrate LGBT rights on Saturday.
It marked 50 years since the Stonewall uprising in New York – a moment which changed the face of the gay rights movement around the world.
Organisers predicted up to 1.5 million people would turn out for the annual march from Portland Place to Whitehall.
Mr Khan said the "huge, huge" progress in gay rights was to be treasured, but said the "heartbreaking" recent images of a bloodied lesbian couple after they were attacked on a night bus were a symbol of the hate still directed towards LGBT people.
Melanie Geymonat and her partner Chris were beaten up by a group of young men for refusing to kiss in May, with the incident sparking a public outcry.
Parade groups marched through streets bursting with colour, music and dance as participants remembered the "people who paved the way for me to feel OK".
Choreographer Ricky Jinks, 24, told PA: "I'm learning more of the history myself. To think it started as a riot and now it's a party is amazing.
"Some people here can remember a time when it was hell for people like us.
"We still have things to overcome and change, but this much changing in 50 years seems like a lifetime, but it's no time at all.
"I'm learning more about the history myself, about the people who paved the way for me to feel OK."
Speaking ahead of the parade, Mr Khan told PA: "You've got the next prime minister using homophobic language.
"When you speak to members of the LGBTQ+ community, they will tell you that some of the homophobia they suffered, the attackers used this language, the same sort of language he's used.
"What I want to see from our prime minister, if it is Boris Johnson – it looks like it will be – is him realising that language matters."
Mr Johnson has been criticised for remarks including calling gay men "tank-topped bumboys" in a 1998 Telegraph column unearthed by the Business Insider website.
He also attacked "Labour's appalling agenda, encouraging the teaching of homosexuality in schools, and all the rest of it" writing in The Spectator in 2000.
Leadership contenders Jeremy Hunt and Mr Johnson took the chance to tweet about the event, with Mr Hunt saying: "Happy Pride! It's so important that we make everyone in our country feel safe and able to express themselves".
Mr Johnson said: "Salute all those celebrating #PrideInLondon today.
"Britain leads the world in LGBT+ equality and I'll continue to champion the cause if I am lucky enough to become our country's Prime Minister."
Efforts to cater for everyone included viewing platforms for the Trafalgar Square stage, sign language interpreters and captioning for all performances across two large screens and accessible, gender-neutral toilets.
Clare Kempster, 44, from Kent, was in a party of five, including two people with disabilities, at Trafalgar Square.
She told PA: "Access has been absolutely wonderful. The volunteers have been fantastic."
Julie Burr added the party would have "struggled" to access the event without provision for disabled people.
Joseph Canestrala, who was dressed in an inflatable unicorn costume, told PA: "I'm from southern Italy and the mentality there is so closed. I came to London five years ago because I want to be free."
The chef, 28, added: "You meet more people. It's more open, you can find a partner. Every year I come to Pride. I love it."
Organisers announced a day ahead of the parade that they had declared a climate emergency, responding to demands by environmental protest group Extinction Rebellion, saying they aim to make the event carbon neutral by 2020.
The parade has been free from plastic glitter since 2017 and other environmentally friendly measures include volunteers being given a refillable bottle on the day.
Met Police officer Tatjana Arsoba, who was with her partner Katie in the force's LGBT parade, said it was important to "support communities to show that we're a part of them".
Hundreds of on-duty officers kept the parade safe, with rest days and leave cancelled, and some taking on longer shifts, Mr Khan said.
Scotland Yard commander Helen Millichap said: "We want Pride to be a friendly and safe event for everyone to enjoy.
"We need the public to help us by taking the usual precautions by remaining vigilant and reporting anything of concern to police officers or stewards at the event.
"As with any large event, the Met's priority is public safety and we are working closely with the organisers in the lead-up to Pride to develop our policing plan."