Thousands of Thyssenkrupp steelworkers protested on Wednesday against the German industrial group's plan to merge its European steel operations with those of Tata Steel.
On Tuesday, German labour union IG Metall had demanded that Thyssenkrupp end the merger talks.
"Management should put its merger plans on ice and discuss with us how the group can make progress," said IG Metall's Detlef Wetzel, who is a member of Thyssenkrupp Steel Europe's supervisory board.
The two companies have been talking since last year about a merger they say would support steel prices and raise efficiency by taking excess capacity out of the market. Trade unions fear large-scale job losses and question the logic of a deal.
"I find it intolerable the way that Thyssenkrupp is talking the steel business into the ground," Wetzel said on Wednesday.
"With friends like our management, who needs enemies?" he asked at a demonstration at Thyssenkrupp's steel headquarters in the German city of Duisburg,
IG Metall, which said about 7,500 steelworkers attended the demonstration, fears 4,000 out of the 27,000 jobs at Thyssenkrupp Steel Europe will be lost if the merger goes ahead.
Andreas Goss, head of Thyssenkrupp Steel Europe, denied any such plans. He reiterated that the business planned to cut costs by €500 million over the next three years, which he said would help save jobs.
"There are no plans for job cuts of this order," he told the Westdeutsche Allgemeine Zeitung. "At the moment, we have no plans to close any sites. But of course we have to negotiate if certain areas show no signs of making a profit long term."
He also told the paper that Thyssenkrupp was considering building a new steel plant in North Rhine Westphalia, to serve demand from the automotive industry for hot dip galvanised steel.
"It would be an investment of over €100 million, which would create jobs," he was quoted as saying.
Thyssenkrupp, which also builds elevators, submarines and car parts, agreed in February to sell its loss-making Brazilian steel mill CSA to rival Ternium for $1.3 billion and took a €900 million writedown.
Thyssenkrupp's European steel operations are profitable and considered among the continent's most efficient but the company, which is 15 per cent owned by activist investor Cevian Capital, wants to focus on its capital goods businesses.
Talks with Tata have stumbled on the question of who will assume liability for Tata Steel UK's huge pension fund. Thyssenkrupp has said there are other, unspecified partners with which it could merge its steel business.
Labour bosses also fear that more cuts could be made at Thyssenkrupp's German steel sites to save jobs at Tata's ailing steel plant in Port Talbot, Wales, where a deal has been struck to protect jobs and investment.
"You cannot just say that the UK sites won't be touched but take the axe to Germany," Wetzel said on Tuesday.