Demonstrators have attacked police officers with a petrol bomb and set fire to vehicles in violent confrontations on the streets of Athens during a May Day protest.
Officers in riot gear retaliated with batons before quickly backing off after each repeated wave of attacks.
As tension escalated, officers fired tear gas at protesters who covered their faces with scarves while others dodged petrol bombs.
A Greek national TV van was also set on fire by rioters.
Meanwhile in Thessaloniki, in the north of the country, police have been firing tear gas at demonstrators attacking banks and businesses.
Sky News’ Europe correspondent Alex Rossi, who is in Greece, said: “We were expecting it to turn a bit ugly – and it has.”
The face-offs come amid May Day protests in the Greek capital that started with peaceful marches when people gathered for a rally at the central Syntagma square.
The Greek government is introducing a series of spending cuts to bring the country out of massive amounts of debt – but the Greek people are furious about what they see as draconian measures.
However, the government must introduce severe structural changes to society for an EU and IMF bail-out to succeed.
Some benefits to employees, such as a two-month bonus each year and extra pay for workers employed outside, are ripe for slicing.
Rossi said: “Things are going to get fairly tough here. These people are very, very angry with their government.”
And he went on: “To give you an idea of the seriousness of the situation, the Greek prime minister has described it as a matter of national emergency and he has even compared it to a war-time situation.”
Trouble has erupted across the world during rallies on May Day, traditionally seen as a day of celebration for workers.
Scuffles broke out in Turkey as tens of thousands of people filled a central square in Istanbul for the first May Day celebrations at the site after dozens were killed there 33 years ago.