Greece's civil servants have staged a nationwide strike against planned pension reforms, badly disrupting transport and closing public offices.
Protesters were marching on Wednesday in central Athens
Trade unions say millions of people took part in the 24-hour action, which was marked by large protests in Athens and Thessaloniki.
Greece's parliament is due to vote on a controversial reform bill on Thursday.
It would eliminate most early retirement schemes, merge pension funds and cap auxiliary pensions.
In central Athens, police fired tear gas in after groups of self-proclaimed anarchists threw petrol bombs and rocks.
The anarchists also set fire to rubbish bins and cars, and smashed bank windows, before dispersing, police said.
Some violence was also reported in the northern city of Thessaloniki as protesters set fire to banks and cash machines.
Unions describe the general strike - the third of its kind in as many months - as the biggest protest so far against the pension reform plan.
It follows a wave of stoppages by workers from various sectors:
- an ongoing two-week refuse collectors' strike has left piles of uncollected rubbish rotting on streets
- protest action by power sector workers has caused nationwide blackouts
- a public transport strike on Tuesday created traffic chaos in the capital Athens.
Wednesday's strike is paralysing train and bus services across Greece and confined ships to ports.
A two-week refuse collectors' strike has left piles of rubbish on streets
A number of domestic and international flights were reportedly cancelled, as air traffic controllers also joined the action.
Schools, ministries and banks were closed for the day.
"The participation in the strike is total. We are talking about millions," Spyros Papaspyros, who heads the civil servants' umbrella union Adedy, told Reuters news agency.
Trade union leaders also say the protests could be the largest public gathering in Greece since democracy was restored in 1974.
The government has so far made no comment on the turnout.
"The [pension] law harms all categories of workers, mainly young people entering the workforce who will not have rights," Yiannis Panagopoulos, one of the labour union leaders, told the Associated Press news agency.
"And of course it dramatically reduces pensions for everyone," he added.
Re-elected last September, the conservative government of Prime Minister Costas Karamanlis wants to overhaul Greece's debt-ridden pension system.
It has warned the system faces collapse unless sweeping reforms are implemented.
Under the proposed scheme, more than 100 social security and pension funds will be merged into a handful of funds, cutting administration costs.
The government is also considering raising the retirement age in some sectors, and giving incentives to those who continue working after the retirement age, which currently stands at 65 for men and 60 for women.
The conservatives may only have a majority of two in the 300-seat parliament, but they are confident they will maintain sufficient discipline to pass the legislation, the BBC's Malcolm Brabant in Athens says.
Mr Karamanlis has been in power for four years. Pension reform was one of his main pre-election commitments.
If he fails to push the changes through Parliament, the credibility of his administration will be critically affected, our correspondent says.
Despite the inconveniences, it seems that most Greeks support the strikers.
A survey by the banking unions showed that 71% of the population opposes the pension reforms and 69% supports the strike.