Greek police have thwarted efforts by striking workers to encircle parliament and block access for MPs due to debate new austerity measures.
Thousands of demonstrators have gathered in Syntagma Square in Athens.
Prime Minister George Papandreou is trying to push through fresh policies as part of the conditions for the EU and IMF's bail-out package.
Ports, public transport and banks are badly disrupted as the main public- and private-sector unions go out on strike.
State-run companies have also joined the walkout, while hospitals are only offering emergency care. However, airports are operating normally after air traffic controllers called off their strike.
It is the third general strike in Greece this year.
A top credit agency has cut Greece's rating, making it the least credit-worthy nation out of 131 countries it monitors.
The Greek government said the downgrade by Standard & Poor's - from B to CCC - ignored its efforts to secure funding.
In order for the next tranche of rescue loans to go through, parliament must adopt the new austerity plan by the end of June.
'Fight the battle'
Between 4,000 and 5,000 protesters have gathered outside parliament, reports the BBC's Malcolm Brabant, who is in Syntagma Square.
They were planning to encircle the building to prevent MPs from getting in for the debate.
However, police sealed off the roads leading to the square and created a pathway for deputies.
The Greek demonstrators are calling themselves the "indignants", linking themselves to Spanish anti-austerity protesters who set up camps in Madrid and Barcelona until they were removed by police last month.
The square is awash with Greek and Spanish flags, as well as banners reading "Resist" and the battle cry from the Spanish civil war, "No pasaran" (they shall not pass), the AFP news agency reports.
Previous general strikes have been spoiled by violent outbreaks but the unions will want to ensure that the Indignant movement, which has caught the national imagination, is not tainted by fighting, our correspondent says.
Mr Papandreou faces the risk of a backbench revolt over the plans.
One MP defected from Mr Papandreou's Pasok party on Tuesday, leaving it with only 155 of the chamber's 300 seats.
"You have to be as cruel as a tiger to vote for these measures. I am not," George Lianis, a former sports minister, said in a letter to parliament's speaker announcing his departure from the parliamentary group.
At least one other Socialist MP has threatened to vote against the new programme of cuts and privatisation of state assets.
Another 14 MPs are wavering in their support for the austerity plan, our correspondent says.
The government has appealed for consensus over its proposals, which would see 6.5bn euros (£5.7bn; $9.4bn) in tax rises and spending cuts this year.
"Every Greek, particularly the new generation, demands that we fight the battle with all our power, a battle to avoid a disastrous bankruptcy which will undermine the future of the country," government spokesman George Petalotis told reporters.
"We are fighting the battle to serve the common good, in the most crucial moment in the country's modern democracy."
The EU and IMF are demanding the measures in return for the release of another 12bn euros in aid next month which Athens needs to pay off maturing debt.
