A rainbow flag is being flown at Brighton and Hove Town Hall to mark the third International Day Against Homophobia (Idaho).
Sussex Police is launching a new campaign against hate crime
Later, the gay community is to release 77 Thai "sky lanterns" to represent the countries of world where gay people are still imprisoned and executed.
The seafront event will also include a show by the Rainbow Chorus Choir.
Brighton is known as the "UK's gay capital", having the highest proportion of gay people per head of population.
"We are living in a world where men and woman are still abused, imprisoned or killed by state agents because they happen to be gay," said Cliff James, Brighton Idaho spokesman.
Police forces across the South East are supporting Idaho, which marks the day in 1990 that homosexuality was removed from the World Health Organisation's list of mental illnesses.
In Brighton, Sussex Police is launching a new campaign to tackle lesbian, gay, bisexual and transgender hate crime.
Pc John Ansell, community liaison officer from Kent Police, said hate crime was still under-reported to police.
"There were about 160 incidents report to Kent Police last year, but people still believe incidents will not be taken seriously," he said.
"Or they might have personal reasons - thinking this is the sort of thing they have to accept if they are gay."
Surrey Police is one of the sponsors of an Idaho event at Oatlands Park Hotel, Weybridge.
Supt Kul Verma will be speaking about homophobic hate crime and Supt Nick Dove about neighbourhood policing.