RUESSELSHEIM (Reuters) -- Thousands of Opel auto workers in Germany downed tools on Thursday to show their anger at U.S. parent General Motors Co. for its decision to abandon a plan to sell Opel to Magna International Inc.

"We want Opel to continue to exist," German politician Roland Koch told rallying workers at Opel headquarters in Ruesselsheim. "We will have to fight again with the goal of saving jobs."

Koch, who is premier of Hesse, the German state where Opel is based, was one of the biggest lobbyists for a sale to Canadian supplier Magna and its Russian partner Sberbank.

GM's board this week reversed the U.S. carmaker's decision to sell a majority stake in Opel, angering the governments of Germany and Russia and irking Opel staff in Germany who had pinned hopes on Magna to save as many jobs as possible.

Around half of Opel's 50,000 staff work in Germany.

As anti-American sentiment swelled in Germany, Koch warned GM not to "maximize profits by taking German workers hostage."

GM CEO Fritz Henderson said he is keen to repair relationships with German trade unions.

“We have some fence mending and repairs that need to be done. That's my responsibility and the responsibility of the leadership team and I'm enthusiastic about getting started and doing that,” he told reporters in Detroit on Thursday.

Insolvency threat

Opel staff had agreed with Magna on 265 million euros in annual savings, but Opel labor leader Klaus Franz again rejected cost concessions from workers to help out GM, whom many in Germany blame for mismanaging Opel into losses.

"Workers will not contribute a single cent," he said, accusing GM of "threats, blackmail and intimidation."

Like Magna, GM plans to cut Opel staff by a fifth and has said its European arm faced insolvency if workers do not agree to cost cuts and countries with Opel plants do not contribute to its 3 billion euro ($4.5 billion) restructuring plan.

The reaction in Germany is in sharp contrast to that in Britain, home of Opel's sister brand Vauxhall, where workers have welcomed GM's decision to hang on to its European arm because they believe GM will cut fewer jobs at Vauxhall than Magna.

GM is counting on European countries that host GM plants including Germany, Britain, Spain and Belgium to provide financial support for a restructuring, which aims to cut fixed costs at loss-making Opel by 30 percent.

European Union Industry Commissioner Guenter Verheugen urged Opel nations to stick together to boost their bargaining power.

"If everyone negotiates for himself with Detroit, the Americans will be able to pick the best offers," he told German paper Hamburger Abendblatt paper. "It is questionable whether that would be the most economically sustainable (approach)."

Labor deal torn up

Opel workers across Europe had reached a deal on cost concessions with Magna this week after months of tense negotiations. Franz tore that deal up in disgust when GM reversed course and decided to keep Opel.

GM cited improving business conditions and the strategic importance of Opel, its European backbone, which also provides technology the group uses around the world.

European Union competition regulators on Wednesday urged GM to draft a restructuring plan driven by economics.

GM Europe head Carl-Peter Forster told German newspaper Bild he now expected "massive cuts" at Opel.

"We had negotiated a good restructuring plan which was ready and on the table," the paper quoted him as saying. "Now there is a danger that the sensible distribution of the burden we had agreed will unravel and the process will start all over again. One thing is certain: even with this solution, there will be massive cuts."

John Smith, GM's lead negotiator in the Opel deal, told a conference call on Wednesday the company had narrowly opted to retain Opel, because they believed losing it could have left a strategic gap in its operations.