(Updates number of workers on strike, adds comments from heads of IG Metall and NiedersachsenMetall, details)
BERLIN/FRANKFURT, Jan 16 (Reuters) - Tens of thousands of German industrial workers downed tools on Tuesday in support of trade union IG Metall's demands for a 6 percent pay rise and a right to the first new cut in weekly working hours since the 1980s.
Workers have been staging such warning strikes since last week, a common tactic in sectoral wage negotiations in Germany. About 33,000 workers took part on Tuesday, including 10,000 at Mercedes maker Daimler AG, taking the total to 425,000 since last week.
With Europe's largest economy steaming ahead and unemployment at a record low, Germany's biggest trade union is confident of winning a significantly better deal for around 3.9 million workers in the metal and engineering sectors.
IG Metall said it would decide next Friday, after the current round of wage talks, whether to escalate the dispute into 24-hour strikes.
"The proposal that the employers have made is far from fair," IG Metall chief Joerg Hofmann told a news conference in Frankfurt. "An offer of a 2 percent wage rise is more like a provocation."
IG Metall is demanding that workers should be able to cut their weekly hours to 28 from 35 if they need to care for children, elderly or sick relatives and get the right to return to work full-time after two years.
The union said it had beefed up its strike war chest by 84 million euros last year thanks to membership dues that rose 2 percent to 561 million euros.
"We have plenty of staying power," said treasurer Juergen Kerner. "Our strike coffers are well filled."
Flash strikes also took place on Tuesday at Schaeffler , Bosch Rexroth, ZF Friedrichshafen , Siemens, BMW and Osram.
In the northern state of Lower Saxony, employers' association NiedersachsenMetall said a third round of negotiations had ended on Tuesday without a result.
"We've offered the trade union alternative solutions but unfortunately we're not seeing much movement from the other side yet," said NiedersachsenMetall head Volker Schmidt.
Roman Zitzelsberger, head of IG Metall in the southwestern state of Baden-Wuerttemberg, told striking workers at the gates of Daimler's factory in the town of Sindelfingen, near Stuttgart: "The economy is booming because the employees like those here at Daimler do good work every day."
"So 6 percent is appropriate and we won't be fobbed off with 2 percent."
Employers reject the demands to cut hours unless working time for others could be increased temporarily as well. They argue that workers in Germany's industrial sector already have shorter weeks than their peers in other countries and worry that reducing their hours further would hurt German competitiveness. ($1 = 0.8191 euros) (Reporting by Michelle Martin in Berlin and Georgina Prodhan in Frankfurt; Writing by Michelle Martin; Editing by Peter Graff)