0546 GMT February 18, 2018
Around 12,000 people took part in separate protests in Athens and another 6,000 in Thessaloniki according to police, a day before the measures are to be approved by parliament, AFP reported.
During the Athens protest, hooded youths smashed the windows of a TV van and threw a firebomb at police, who responded with bursts of tear gas.
The walkout cut off maritime traffic for a second day while dozens of flights had to be canceled or rescheduled and hospitals ran on emergency staff.
"We want to send a decisive message to the government, the European Union and the International Monetary Fund that we will not let them cut up our lives," Alekos Perrakis, a senior member of Communist union PAME, told reporters.
The mobilization is aimed against new pension and tax break cuts forced on Greece by its EU-IMF creditors in return for bailout cash.
Overall, the bill to be approved on Thursday night entails 4.9 billion euros ($5.4 billion) in cuts in 2018-2021, state agency ANA said.
The leftist government of Prime Minister Alexis Tsipras grudgingly accepted to legislate another round of pension cuts and lower tax breaks -- applicable in 2019 and 2020 respectively -- to unlock the cash payment ahead of looming debt repayments in July.
The tax break cut on its own "is tantamount to losing one-and-a-half (months') salary," Perrakis said.
In return, Greece will enact poverty support measures – such as subsidies on rent and medicine – over the same period of time.
Police unionists unfurled a huge black banner from Lycabettus Hill atop Athens in German and Greek, addressed to German Chancellor Angela Merkel and Tsipras.
"What is the life of a police officer worth?" it asked, referring to the cuts.
Under Merkel and her Finance Minister Wolfgang Schaeuble, eurozone powerhouse Germany has been pushing Greece for the past seven years to make recurring cuts in return for bailout cash.
Germany has footed an important share of Greece's three rescue bailouts since 2010.
Athens hopes that the loan payment will be approved by a meeting of eurozone finance ministers on May 22.
It also expects a clear pledge later this month on measures to ease repayment on its huge public debt, which represented 179 percent of annual output at the end of last year.
Athens also hopes to be finally allowed access to the European Central Bank's asset purchase program, known as quantitative easing, or QE, to help its return to bond markets.
There is speculation that Greece plans to issue a three or five-year bond in July.
The Finance Ministry declined to comment on the issue on Wednesday.