Hundreds of shipyard workers and supporters filled the square outside Victoria Gate to protest job losses announced last month by defence giant BAE systems.
More than 900 jobs will be lost, although Portsmouth will retain repairs and maintenance work.
Unite national officer Ian Waddell said although he expected the cuts would be final, unions wanted to send a clear message to government that shipbuilding jobs in Portsmouth and surrounding areas like Southampton needed to be protected.
"It'd be a devastating blow if this industry was to go," he said.
"It'd be the end of 500 years of maritime history in this area.
"There were people in the crowd today who had the indenture papers of their fathers and their grandfathers - these are really jobs that have been handed down through the generations."
The cuts come as about 830 jobs will also be lost at yards in Govan and Scotstoun, on the River Clyde in Glasgow, at Rosyth in Fife, and BAE's Filton office, near Bristol, as a result of a drop in work after the end of aircraft carrier work.
Mr Waddell said he could understand why BAE needed to let staff go, given the lack of Ministry of Defence work would be insufficient to sustain the amount of shipyards the region had.
"But what we are confident about is there are a whole array of complex ships and vessels that need to be built in the commercial sector," he said.
"There's a market for that and that's exactly the sort of work that could be done out of Portsmouth if the right conditions were created."
Mr Waddell said job losses would not only affect shipyard workers.
"We were calling on the communities of Portsmouth, Southampton and the whole south coast to support the workers in the shipyard because for every one job that's lost here, it has a knock-on effect in the rest of the community - in pubs, in shops and restaurants," he said.
"This really is about the south coast fighting to save the shipping industry."