(Recasts throughout, adds Petrobras details, Supreme Court decision on Temer)
By Pedro Fonseca
RIO DE JANEIRO, June 30 (Reuters) - Labor unions staged a nationwide strike on Friday to protest against legislative changes to Brazil's labor and pension laws that are central to the economic reform agenda of embattled center-right President Michel Temer.
Subway and bus services shut down in Brasilia, the nation's capital, while demonstrations blocked roads and snarled traffic in the megacities of São Paulo and Rio de Janeiro as union activists took to the streets.
However, the protest appeared to have limited impact and triggered none of the violent clashes between police and protesters that marked a much larger strike in April.
It came after Brazil was riveted this week by a corruption charge filed against Temer by the country's top prosecutor.
The charge, the first ever leveled against a sitting president in Brazil, marked a milestone in a three-year probe by investigators that has revealed stunning levels of corruption in Latin America's largest country.
Temer, one-third of his cabinet, four past presidents and dozens of lawmakers are either on trial, facing charges or under investigation for corruption. Over 90 people have been found guilty so far.
Brazil's largest oil workers federation said in an emailed statement that Friday's work stoppage would continue for an indefinite period, and that all 10 refineries where it represents workers were affected.
Executives at state-controlled oil company Petróleo Brasileiro SA acknowledged that the job action had a limited impact at oil refineries. But they said exploration and production activity, along with logistics, carried on as normal.
Temer, who replaced impeached leftist President Dilma Rousseff last year, was charged with graft on Monday by Prosecutor General Rodrigo Janot after executives of the world's biggest meatpacker, JBS SA, accused him of taking millions in bribes.
He has denied any wrongdoing and resisted repeated calls to resign. But the lower house of congress is preparing to vote on whether he should face a trial in the Supreme Court, which would prompt his removal from office for at least 180 days.
Other criminal charges against Temer are widely expected to be filed by Janot, and a ruling on Friday by Supreme Court Judge Edson Fachin appeared to come in anticipation of that.
Fachin said each and every charge against the president would have to be investigated separately, meaning that Temer could potentially face more than one trial before the court.
Unions fiercely oppose Temer's labor reform bill as it reduces their power over workplaces by cutting mandatory dues and allowing companies and employees to negotiate contract terms more freely. The bill has already been approved by the lower house of Congress and will likely pass the Senate within a few weeks.
Unions also criticize Temer's pension overhaul proposal as it would make Brazilians work more years before retiring.
Economists and investors see pension reform as the only way for Brazil to shore up its finances in the long run without resorting to tax hikes. (Reporting by Pedro Fonseca in Rio de Janeiro and Ricardo Brito in Brasilia; Writing by Silvio Cascione; Editing by Marguerita Choy and Tom Brown)