The Pride in London showpiece parade will be launched by the emergency services in honour of their bravery shown in recent tragic events, organisers said.
The festival said London's Fire and Rescue service, the Metropolitan Police and London Ambulance service are all nominating LGBT+ colleagues to officially open the parade at 1pm on Saturday.
Colleagues who were involved in the Grenfell Tower fire, the London Bridge terror attack and the Westminster terror attack will be joined by flag bearers showing flags from countries around the world, including countries where it is still illegal to be LGBT+.
An estimated 26,500 people are due to take part in the parade and there is expected to be one million people in attendance.
Revellers in the parade will take a 1.4 mile (2.3km) route starting north of Oxford Circus on Regent Street.
The parade comes after what is believed to be the world's largest Pride festival, with over 100 events since Saturday June 24.
As part of the annual Pride weekend, a rainbow flag will be projected on to the Palace of Westminster for the first time.
Lord Fowler, Speaker of the House of Lords, said: "Homosexuality is still illegal in over 70 countries around the world, including many in the Commonwealth.
"None of this will be solved by a march, or a display of lights in Westminster.
"But these acts will demonstrate to those who are being persecuted or abused that they are supported.
"The lights are a symbol of our support."
The Metropolitan Police said officers from the police liaison team will be on the ground to engage with the organisers, and there will also be high visibility police patrols, including both armed and unarmed officers, as well as plain clothes officers.
The force said it encourages those who wish to show their support for the LGBT+ community to come and watch the parade, and added that the current threat level remains at severe, asking all attendees to be vigilant and report anything suspicious to police or stewards.
Chief Superintendent Helen Millichap, Met spokeswoman for the event, said: "We know that recent events in London and Manchester will cause people to worry.
"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.
"We want Pride to be a friendly and safe event for everyone to enjoy and, to help us, we need the public to take the usual precautions by remaining vigilant and reporting anything of concern to police officers or stewards at the event."
The parade is also marking 50 years since the Sexual Offences Act 1967, which decriminalised homosexuality in England and Wales.
Society has come a long way since then but there is more to be done, said Lord Paddick, who was the most senior openly gay police officer in the UK before retiring from the Met.
He told BBC Breakfast: "A lot of people on the parade, maybe most of the people on the parade, weren't even born 50 years ago. I was and, you know, it was a step in the right direction. There's been a lot of movement in terms of the law since then.
"We now have same sex marriage. It's not equal marriage, there are still some anomalies for transgender people that need to be addressed through legislation, and of course people in Northern Ireland still don't have same-sex marriage."
He said some gay people in the UK still face not being accepted for who they are.
"I think as more people are more open about their sexuality they become a target for people who are homophobic," he said.
"We still have youngsters being kicked out of their home by their parents in the UK, in 2017, just because they are gay. That is the reality.
"There is a long way to go in terms of attitudes, even if the legislation has progressed."