Police have fired tear gas in running battles with stone-throwing youths in Athens, where a 48-hour general strike is being held against a parliamentary vote on tough austerity measures.
Thousands of protesters gathered outside parliament in the capital where public transport ground to a halt.
PM George Papandreou has said that only his 28bn-euro (£25bn) austerity plan would get Greece back on its feet.
If the package is not approved, Greece could run out of money within weeks.
Without a new plan in place, the EU and International Monetary Fund say they will withhold 12bn euros of loans which Greece needs to repay debts due in mid-July.
The newly named IMF chief, Christine Lagarde, has urged Greek politicians to unite to avoid a debt default.
"If I have a message this evening about Greece, it is a call to the Greek opposition... to join in national unity with the party which is currently in power," she told France's TF1 television station. "The country's destiny is at stake."
As thousands of peaceful demonstrators observed a night-time protest outside the Greek parliament on Syntagma Square, sporadic clashes continued nearby between black-hooded, rock-hurling youths and police firing tear gas.
Earlier, more than 5,000 police officers were deployed in the city centre to monitor what started off as a peaceful rally, but rapidly deteriorated into running skirmishes on the fringes of the main demonstration.
Hundreds of protesters - their faces covered by scarves or gas-masks - started throwing stones, debris and bottles in the square at police who retaliated with tear gas and stun grenades.
Two communications vans with mobile telecoms transmitters were daubed with graffiti condemning the media and banks before being set alight by protesters who had apparently mistaken them for satellite TV trucks.
As many as 20 police officers and four demonstrators were injured in the scuffles, police said, while a number of demonstrators were treated for breathing difficulties.
Some 18 people were detained by police, Reuters reported.
There were also skirmishes as trade unionists tried to persuade anarchists to leave the square, saying their violent protests were only harming the aims of the demonstrations, says the BBC's Jon Sopel in Athens.
Greece's general strike has halted most public services, banks are closed and hospitals are operating on skeleton staff.
Airports are shutting for hours at a time, with air traffic controllers walking out between 0800 and 1200 (0500-0900 GMT) and 1800 and 2200 (1500-1900 GMT).
A number of flights were cancelled at Athens international airport while trains, buses and ferries were also affected.
The capital's underground system was the only form of public transport working "to allow Athenians to join the planned protests in the capital", metro drivers said.
Protesters blockaded the port of Piraeus, near Athens, which links most Greek islands with the mainland.
"The situation that the workers are undergoing is tragic and we are near poverty levels," said Spyros Linardopoulos, a protester with the PAME union at the blockade.
"The government has declared war and to this war we will answer back with war."
The unions are angry that the government's austerity programme will impose taxes on those earning the minimum wage, following months of other cuts which have seen unemployment rise to more than 16%.
Polls suggest that between 70% and 80% of Greek people oppose the austerity plan.
"We know very well that these measures will be our tombstone," said bank worker Kali Patouna.
"They will have extreme consequences for workers and for everyone on all social levels."
The austerity package and implementation law must be passed in separate votes on Wednesday and Thursday.
If the measures are passed, the next instalment of Greece's 110bn-euro bail-out will be released by the European Union and International Monetary Fund.
European officials will also start to finalise the details of a second bail-out - worth an estimated 120bn euros - designed to help Greece pay its debts until the end of 2014.
EU President Herman Van Rompuy said the impact of the Greek vote would be felt worldwide.
"The coming hours will be decisive, crucial for the Greek people, but also for the eurozone and the stability of the world economy," Mr Van Rompuy told the European parliament.
A Greek government defeat would send ripples of anxiety right across the eurozone, with Greece facing the prospect next month of becoming the first member state to default on its debts, says the BBC's Chris Morris in Athens.
Mr Papandreou has warned that failure to secure the new loans would mean that national coffers could be empty within days.
The recently-appointed Finance Minister Evangelos Venizelos acknowledged that the cuts were "unfair", but said they were absolutely necessary.
He called on MPs to back the measures, saying both the government and the opposition were "running out of time".
But the main opposition leader, Antonis Samaras of the New Democracy party, said the thinking behind the austerity package was flawed and that tax rates should be lowered rather than raised in order to stimulate the economy.
The outcome of the debate is uncertain. Mr Papandreou faces opposition from within the governing Panhellenic Socialist Movement (Pasok), with two MPs saying they may oppose the bill.
The party has a slim majority, with 155 seats out of 300 in parliament.