James Cummings and Fred Starrett died when a booby-trap bomb detonated behind hoardings at Royal Avenue, as Castle Court was being built, in February 1988.
The fatal attack is marked every year.
Marchers left Templemore Avenue in east Belfast at around 10am on Saturday morning, passing St Matthew's Church.
It is understood some of the bands could be heard playing the Sash.
The event culminated with a service of commemoration outside Castle Court, which was attended by the families of Mr Cummings and Mr Starrett.
It means a lot and I'm very thankful for everyone who turned out. You never really get over it ... you just learn to live with it.
Floral tributes were laid and Reverend David McIlveen spoke to those who gathered.
He said: "Today, 25 years on from the murder of their family members, we are here to say our thoughts and prayers are very much with them".
Neil Cummings, James' brother, said the pain of their loss has never left the family.
"Today is a very special day for the families that everybody came along," he told UTV.
"It is a great loss. When something like that happens to a family the family is never the same, it just wrecks a family."
There was a heavy police presence as the Orange march - the first of the marching season - returned to east Belfast, passing the nationalist Short Strand interface without incident.
Earlier organisers called for dignity for everyone involved in the first official parade of the marching season.
The Secretary of State for Northern Ireland, Theresa Villiers, also appealed for calm.
"I urge everyone to play their part in ensuring today's parade passes peacefully," she said.
"This is an occasion which should be commemorated in a respectful way, and the rule of law must be upheld."
Meanwhile the weekly loyalist flag protest has taken place outside PSNI headquarters at Knock Road, also in the east of the city, after it was moved from City Hall.
A small number of people came together to complain about how the police have handled the demonstrations, which have been taking place since last December when the council voted to stop flying the Union flag all year long.
Another small protest was held near City Hall, a short time after the Orange Order march through the city centre.