Some 15,000 people protested in Athens on Thursday, as unions went on a general strike against a scheduled social security overhaul, making it the second walkout in less than a month.
The demonstration took place in a calm atmosphere, until youths lobbed a few firebombs towards riot police outside the Finance Ministry and the parliament.
As the police responded with stun grenades, the young Greeks retreated and the violence did not continue.
Lawyers and teachers also took part in the walkout, while journalists went on a solidarity strike on Wednesday.
The protests came as the Greek Labour Ministry is working on a new system with the intention to cut state-guaranteed pensions reportedly by half , which is a minimum of 384 euros and the rest will depend on the income and years of social security payments.
A similar general strike was held on November 12, with the participation of 20.000 people in Athens against the EU bailout and accompanying tax hikes.
The Greek government has already been weakened by its decision in July to accept a third EU bailout with strict austerity conditions to prevent Greece from exiting EU.
Greek Prime Minister Alexis Tsipras finds himself in an unstable political position, because his leftist-nationalist coalition in November saw its parliamentary majority drop to 153 deputies in the 300-seat parliament, after a disruptive vote on home foreclosures, which was another precaution demanded by international creditors.
Tsipras on Tuesday dismissed suggestions that his government is on its last legs saying, "Dogs howl but the caravan goes on."
"We will endure and achieve our targets. The people will judge us at the end of our term...in the autumn of 2019," he said.
Tsipras’ Syriza party which has won three successive ballots since 2014, applauded protests against the EU bailout, which the PM has called a "tactical retreat."
Head of the leading civil servants' union Nikolaos Adamopoulos suggesting a similar fate for Tsipras says opposing the bailouts has already collapsed three governments.
"There will be a rearrangement in the political scene. There may be a majority in parliament in favour of the bailout but this is not reflected in society," Adamopoulos said
"The people do not trust parties, they showed it in the last election," he added.
In the September election, nearly one in two Greeks eligible to vote did not go to the polling stations.