Thousands of steelworkers hit the streets of Bochum in western Germany on Friday in opposition to industrial conglomerate ThyssenKrupp's plans to merge its steel division with India's Tata.

"We aren't convinced that merging with Tata is the best and most appropriate path for ThyssenKrupp Steel," said Knut Giesler, a regional leader of powerful union IG Metall.

ThyssenKrupp, whose products range from steel to car parts, elevators and submarines, said Wednesday it had signed a memorandum of understanding with Tata to create a joint venture in 2018 that would unite their European steel arms.

The two firms hope to boost their resilience against a flood of subsidised Chinese steel that has roiled western markets.

But politicians and worker representatives were quick to take up arms against the scheme.

"We need a solid future strategy for ThyssenKrupp and not a 'bad bank' for steel," Giesler urged, referring to structures created to help wind down high-risk assets after the financial crisis.

Top of unions' priorities are the 4,000 jobs losses managers said would be shared roughly equally among the two groups when they merge, with no clarity so far on exactly where the axe would fall.

German employees are also concerned about bosses' plan to base the new joint venture in the Netherlands, which they fear could take decisions about the future of steelworking out of their hands.

Employee participation is of "crucial importance" and "must not be traded away", Social Democratic Labour Minister Andrea Nahles told the workers' rally in Bochum -- just two days ahead of Sunday's general election.

ThyssenKrupp remains a stronghold of the post-war German social model that gives workers a voice in strategic decisions.

While bosses have the decisive say at most big companies, steel firms have a strict 50-50 division of seats between workers and executives on their boards