Tens of thousands of people are due to join the Pride parade through central London later.
The annual march, now in its 45th year, will mark 50 years since homosexuality was decriminalised in the UK.
The colourful parade will begin at Portland Place and pass through Oxford Circus and Regent Street, before ending in Whitehall.
A rainbow flag will be projected on to the Palace of Westminster for the first time in celebration of the event.
A number of stores and tube stations in London have been decorated with signs and posters using the colours of the LGBT flag.
The Parliament building will also be lit up in the Pride colours as part of the celebrations.
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.”
More than 150 police officers will take part in the parade, alongside representatives from the London Ambulance Service, London Fire Brigade and British Transport Police.
The Met said it had been working closely with Pride in London and a detailed policing plan had been put in place.
The force added: “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.”
The first official Pride march took place in 1972 and saw 2,000 men and women take part.
Last year’s event saw more than 40,000 people join the march.