Unions and opposition parties say government plans to overhaul labour laws undermine collective bargaining and formalise overtime exploitation by employers.
Thousands of Greeks have marched in Athens against government plans to overhaul labour laws which unions say will undermine workers' rights and let companies bring in longer hours through the back door.
Greece's biggest private and public sector unions staged a day-long strike that brought public transport to a standstill and kept ferries docked at ports.
"It won't pass," public sector union ADEDY said.
At least 10,000 people joined the rally outside parliament, which is expected to vote on the bill this month.
In announcing the bill in May, Prime Minister Kyriakos Mitsotakis' conservative government said the measures would modernise "antiquated" laws dating back decades to a pre-internet time when most workers clocked into their offices and factories at the same set hours.
It said the changes would offer workers more flexibility to decide their own schedules and help tackle exploitation, unpaid overtime and undeclared work which have fuelled a large, untaxed grey economy.
The most contentious part of the bill allows employees to work up to 10 hours on one day and less time on another. Unions fear that will enable employers to force workers to accept longer hours.
'Hands off the 8-hour workday'
A banner unfurled near the Labour Ministry in central Athens read "Hands off the 8-hour workday."
"No matter what the government does, this bill is condemned by workers," Dimitris Koutsoumbas, head of the communist KKE party, told reporters.
"It belongs in the trash heap," he said.
Another 10,000 marched in Thessaloniki, and protests were held in other major Greek cities.
Critics have labelled the reform, which promotes working hour flexibility, "modern-day slavery".
Critics say the government is seeking to raise the eight-hour working day to 10 hours and scrap the five-day working week and collective bargaining agreements, as well as make it harder to call strikes.
The left-wing Syriza party said the bill was a "monstrosity" and called on the government to withdraw it.
"They will not take us back to the 19th century," Syriza said.
The bill would also give workers the right to disconnect outside of office hours and introduce a "digital work card" to monitor employees working hours in real time, as well as increase legal overtime to 150 hours a year.