Public transport workers in the Greek capital have walked off the job in a 24-hour strike, causing traffic jams around Athens, while a doctors' strike has left hospitals functioning with emergency staff.
Athens was without buses, suburban rail and trams, while dock workers began a 48-hour strike that will leave islands without ferries for two days. Hospital doctors and ambulance workers are also on strike for 24 hours.
The civil servants' union has declared a three-hour stoppage at all public services from noon in solidarity, which was expected to shut down post offices, tax offices and other services.
Greece has been gripped by a severe financial crisis since late 2009 and is being kept afloat by billions of euro in rescue loans from other eurozone countries and the International Monetary Fund.
In return, the government has imposed waves of spending cuts and tax hikes, leading to severe salary and pension cuts and leaving unemployment spiralling above 26%.
Workers have been protesting against planned reforms to the pension and income contribution system, part of the latest spending cuts in the bailout programme.
Other measures that took effect this year include a 25% cut to the incomes of most public sector workers, leading to a string of new strikes and protests.
On Wednesday, protesters from a communist-backed union forced their way into the Labour Ministry, leading to the arrest of about 35 protesters and clashes with riot police who tried to repel a crowd of about 200 outside the building.
The arrested unionists are also due to appear in court on charges of causing damage inside the ministry, which they deny.