* Workers strike at firms including Porsche, MAN
* More than 15,000 have taken part in stoppages so far
* IG Metall union plans further strikes on Tuesday
* Next round of talks with employers start on Thursday
* German economy buoyant, record numbers in work (Adds employers' association head, more strike participants)
DUESSELDORF/HAMBURG, Jan 8 (Reuters) - Thousands of German workers staged strikes at metals and engineering firms on Monday in support of wage claims by union IG Metall, which announced a further wave of walkouts for Tuesday.
With the economy in robust health and unemployment at record lows, the country's biggest union is demanding an inflation-busting 6 percent pay hike this year for about 3.9 million workers.
Ahead of a round of regional negotiations due to begin on Thursday, employers have so far offered 2 percent plus a one-off 200 euro ($240) payment in the first quarter.
On Monday, more than 3,000 employees downed tools at sports car maker Porsche in Stuttgart, taking the number of industrial workers who have taken action since the start of last week to more than 15,000, according to IG Metall.
"The employers provoked these warning strikes with their behaviour at the negotiating table," Knut Giesler, head of IG Metall in the state of North Rhine-Westphalia, Germany's most populous.
In that state, the country's industrial heartland, the union has called for stoppages at 143 companies on Tuesday, including Lumileds, the LED components and car lighting business formerly owned by Philips, and Thyssenkrupp unit Rothe Erde.
IG Metall is also campaigning for a right to reduce weekly hours to 28 from 35, and return to full-time employment after two years, for shift workers and those caring for children or other relatives.
Employers have rejected that proposal too.
"If we provide an incentive to work less then we won't be able to keep output at the current level," Oliver Zander, chief executive of the Gesamtmetall umbrella association of employers that mirrors IG Metall, told Reuters.
A proposal by IG Metall that employers help make up the pay shortfall for low-paid workers who need the extra time to care for relatives could be a dealbreaker in negotiations, he said.
In North Rhine-Westphalia, night shift workers at automotive supplier Kirchhoff in North Rhine-Westphalia walked out, as did staff at farming machinery maker Claas, the union said.
In Berlin, around 400 workers at elevator maker OTIS downed tools, along with others at dozens of other companies across Germany including truck maker MAN and automotive supplier Wabco.
On Tuesday, industrial action is also scheduled in Bavaria, home to premium carmaker BMW and engineering group Siemens, where the union is threatening to shift from short warning strikes lasting only a few hours to full-day stoppages.
The nationwide dispute follows a strong year of growth in Europe's largest economy, driven by domestic demand from record numbers of German workers while borrowing costs and inflation remain low and exporters benefit from a global recovery.
That pattern should extend through 2018, with the Ifo economic institute last month forecasting growth of 2.6 percent for the year.
Talks between unions and employers' associations are set for Thursday in the southwestern state of Baden-Wuerttemberg, where Volkswagen's Porsche, Mercedes-Benz maker Daimler and automotive suppliers including Bosch are based.
Next door in Bavaria, negotiations will resume on Jan. 15, and to the north in North Rhine-Westphalia on Jan. 18.
The latter state's sector employers' association, Metall NRW, said the strikes risked damaging Germany industrial companies' reputation as reliable suppliers.
"I expect the union to take action at the negotiating table and not out in the streets," Metall NRW director Luitwin Mallmann said in a statement.
$1 = 0.8299 euros Reporting by Matthias Inverardi and Jan Schwartz; Additional reporting by Alexander Huebner and Ilona Wissenbach; Writing by Maria Sheahan; Editing by John Stonestreet