Children and their families took part in the demonstration, which saw them walk from the Royal to gather at the Public Health Agency building at Linenhall Street on Saturday morning.
The march is part of the Hands Up for Heart Surgery campaign.
Concerns about the future of paediatric cardiac surgery at the Royal were first raised back in August last year, when Health Minister Edwin Poots confirmed that the service was under consideration.
A review had found that although the unit at the Royal - which sees around 90 operations carried out annually - was above par, it would not be realistic to maintain.
If the provision of paediatric congenital cardiac surgery is withdrawn, young patients across Northern Ireland may have to travel to England for treatment.
I want them to keep it here, so the other babies who have to have heart surgery don’t have to move.
Ruth, age 7
One mum told UTV just why it was so important for her to turn to support the services under threat.
"Our daughter was diagnosed with a congenital heart condition last year and had open heart surgery just in October past," Liz Breen said, praising all the staff involved at the Royal.
"It would just be terrible to see a service like that - an amazing service - leave Northern Ireland."
A number of Stormont MLAs were also present at the rally on Saturday.
"I know how important it is to have specialist services close by when you're very young and vulnerable," the SDLP's Conall McDevitt said.
"That's what we need to protect here - our services. And we need to protect them in the context of an all-island service."
Sinn Féin's Sue Ramsey also called for the health minister to consider an all-island approach, to spare children the journey overseas for treatment.
"One of the key issues that came out of the review was that air transport shouldn't be used as an option," she explained.
Alliance's Kieran McCarthy added: "If devolution is to mean anything then our local health minister must make a decision that is in the best interest of people in Northern Ireland, and not just follow a UK wide decision."