The LGBT+ festival, he added, mixes party with protest.
“We do party in Derry as well as anybody else but the focus is always on the political," he said.
"There is a lot of stuff that has to be achieved yet on these islands but internationally there is a lot of awful stuff happening; in so many countries homosexuality is illegal.
“The death penalty exists in Uganda, for example. We see ourselves in an international context. We are marching for those people.”
'Celebration of diversity'
The festival has grown in the last 30 years to become one of “the key dates in the city’s calendar”, Mayor of Derry and Strabane Lilian Seenoi-Barr said.
“It’s a real celebration of diversity and individuality, the parade embodies inclusion and the atmosphere on the day is electric,” she added.
The parade leaves the North West transport hub at 14:00 BST before crossing the Craigavon bridge, and making its way through the city centre.
A family fun day and a Guildhall Square party are due to take place following the parade.
Police have advised motorists of some traffic disruption but say officers will “be on the ground to accommodate the flow of traffic”.
Among the festival's closing events on Sunday is a picnic remembering
Mr Ashton grew up in the County Antrim town of Portrush before moving to London in 1978.
He was a leading figure in the Lesbians and Gays Support the Miners (LGSM) group during the 1984 miners' strike.