Greek riot police have clashed with protesters in Athens and the main port of Piraeus during a 24-hour general strike against cost-cutting reforms.
In the capital, demonstrators threw sticks, bottles and stones at police who responded by firing tear gas.
Tear gas was also fired at ship workers trying to blockade Piraeus's port when they swarmed aboard an arriving boat.
The strike comes as MPs debate tough austerity measures demanded by the International Monetary Fund and EU.
They include cutting pensions, raising the retirement age and making it easier for companies to dismiss employees.
Greece has been suffering a severe economic crisis, and the reforms were a condition of a 110bn-euro (£89bn) international bail-out.
After more than six months of austerity measures and industrial strife, the confrontation between the government and the trade unions is reaching a climax, says the BBC's Malcolm Brabant in Athens.
The debate in parliament over proposed reforms that began on Tuesday is expected to last more than a week.
The challenge for the trade unions is to get as many people on the streets as possible to convince potentially rebellious Socialist MPs to vote against their own party and defeat the bill, our correspondent says.
The government holds a seven-seat majority in the 300-member house.
During a demonstration by about 10,000 people on the streets of the capital on Tuesday, some hooded and masked youths threw stones and other missiles at riot police, who responded with tear gas and stun grenades.
After the youths dispersed, they continued to clash with police along main streets running through the city centre for several hours. Bus stops and phone booths were smashed, and the windows of shops and banks broken.
Seven police officers were injured, while six demonstrators were arrested.
Tuesday's strike shut down many public services and disrupted local transport. Medical workers and journalists took part, meaning that hospitals were running on emergency staffing and TV news programmes were cancelled.
Some domestic flights were cancelled but international flights were unaffected.
The strike in the nearby port of Piraeus - like a similar protest last week - was declared illegal, and the authorities tried to prevent Communist-affiliated unionists from blocking the departure of ships to Aegean islands.
Ferries did leave the port early in the morning, though there were angry scenes as some passengers were prevented from boarding and other ships were blocked. Officials said their tickets would be valid on Wednesday.
"They want to put us in a straitjacket so we work for free all our lives so that some can have their wealth and get very rich at our expense," Sotiris Poulikogiannis, a protester in Piraeus told the Associated Press.
"We don't accept this. Day by day we'll grow stronger and more aware of how to overturn this situation."
One tourist told the BBC that he had tried to catch a boat from Piraeus after his flight from Santorini to Athens was cancelled, but that police had ordered passengers away from the port.
"Much as I enjoyed my holidays in Greece, this experience has left a very negative impression and it will serve only to turn off people wanting to visit the country," said Lay-Kok Tan.
The tourist industry was in uproar after the government did not enforce the court order declaring last week's strike in Piraeus illegal.
It claimed that such confrontations did irreparable damage to Greek tourism, which generates almost 20% of national income.
Strikes against austerity measures have brought the country to a standstill on several occasions, closing airports, roads and railways.
In a major protest on 5 May, three people were killed in the fire-bombing of an Athens bank.