Workers and their families from Tata's Orb steelworks in Newport took over the city centre as they protested at the decision to shut the plant.
The Indian steel giant announced last month it was shutting the works, which makes specialist electrical steels, as it had been unable to find a buyer and the plant needed £50m of investment to make it competitive.
Supporters from other steelworks across the UK, including Rotherham, Shotton and Scunthorpe also joined the protest.
The protesters carried banners provided by the Community Union saying "Save Orb Steel".
Many politicians were present including Jessica Morden MP and Newport East AM John Griffiths AM, who told the protesters: "We need electrical steel for electric vehicles to be made at the Orb works."
Steve Turner, general secretary of the Unite union: "You should be proud that so many people have turned out here today.
"If they think we are going to sit back and let them close Orb, they can think again."
Newport west AM Jane Bryant in her speech: “What is this government doing, not investing in this industry, not investing in the steel industry in Wales and in Newport.
"Today we send a message of support and solidarity with all the workers To the uk government I say get your act together. Support our steel and our steel workers.”
Roy Rickhuss, General Secretary of the Community Union said: "Let us not let Tata off the hook.
"They have made a good living out of Orb and other works for many years."
