* Tens of thousands take part in strikes on Wednesday
* Union demands 6 pct higher pay, right to shorter hours
* Regional talks in Baden-Wuerttemberg due on Thursday (Recasts, adds comments by IG Metall chief, IG BCE union, Deutsche Post wage demand)
By Tilman Blasshofer and Maria Sheahan
HOMBURG/FRANKFURT, Jan 10 (Reuters) - Powerful German union IG Metall has threatened to call for all-out strikes across the industrial sector if talks with employers over wages and flexible hours scheduled for Thursday fail to make progress.
"We have the tool of 24-hour warning strikes at our disposal, and of course we always have the option to ballot for open-ended industrial action," IG Metall chief Joerg Hofmann told Reuters TV on Wednesday.
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.
But a big sticking point is a proposal that workers should receive the right to reduce their weekly hours to 28 from 35 to care for children or elderly or sick relatives, and return to full-time employment after two years.
Employers have so far offered 2 percent plus a one-off 200 euro ($239) payment in the first quarter and have rejected demands for a shorter work week unless hours could be increased temporarily as well.
They said German industrial workers already worked fewer hours than their peers in other countries, and a further reduction would make Europe's biggest economy less competitive.
Employers' association Gesamtmetall said average working hours at German metals and engineering companies stood at 35.4 hours per week in 2016, compared with 35.8 in France, 37 in Britain and 38.4 in Spain.
In the state of North Rhine-Westphalia, the industrial heartland of Germany, more than 25,000 workers at companies including Thyssenkrupp and automotive supplier Benteler took part in walkouts on Wednesday.
To the south in Bavaria, rallies were planned at firms including engineering group Siemens, and further walkouts are planned across the country on Thursday.
Industrial action is common during sectoral wage negotiations in Germany and took place on a similar scale in 2016 and 2015.
IG Metall is not the only German labour union demanding a big pay hike following a strong year of growth in Europe's largest economy helped by domestic demand from record numbers of German workers.
Chemicals and mining workers union IG BCE said it was time for a redistribution of robust profits.
"When even (chemicals trade body) VCI... says the situation is currently 'unconditionally good', then that means that the ladies and gentlemen there are dancing on the tables," IG BCE President Michael Vassiliadis told journalists late on Tuesday.
Union Verdi is demanding a choice between a 6 percent wage increase or more time off for around 130,000 workers at postal and logistics group Deutsche Post.
Rail operator Deusche Bahn had found last year when it offered a choice between higher pay, shorter hours or more days off that more than half of workers chose to have more days off.
Industrial employers have said they cannot make up for lost production if staff shorten their hours because they cannot find skilled workers. They have also rejected the idea that they should help make up for the pay shortfall for some workers who shorten their hours.
IG Metall has meanwhile made clear that it is drawing a line in the sand regarding the right to shorter working hours, saying employers had a social responsibility to contribute to issues related to health and family.
"We want (shorter hours) to be possible for everyone, including those at the bottom of the income scale. That is why we need pay to be subsidised, so that children, caring for family and health don't depend on how much you have in your wallet," Hofmann said.
Talks between IG Metall and the regional employers' association 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.
$1 = 0.8377 euros Reporting by Maria Sheahan; Additional reporting by Matthias Inverardi, Ilona Wissenbach and Jan Schwartz; Editing by Ludwig Burger, Jeremy Gaunt and Peter Graff