Greece's civil servants are staging a nationwide strike against planned pension reforms, badly disrupting transport and closing public offices.
Trade unions say millions of people are taking part in the 24-hour action, which they describe as the biggest protest so far against the plan.
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.
The action is the country's third general strike against the proposed reforms in as many months, and 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 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.
In Athens, thousands of people took part in a protest rally, and demonstrations were held in several other cities.
Trade union leaders 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.
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