Workers in Greece are due to stage a general strike, as the parliament meets to debate new austerity measures.Demonstrators say they will encircle the parliament building in an attempt to prevent MPs from taking part.
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.
A top credit agency has cut Greece's rating, making it the least credit-worthy country of 131 it monitors.
The Greek government said the downgrade by Standard & Poor's - from B to CCC - ignored its efforts to secure funding.
'Fight the battle' Activists and unionists plan to gather at Syntagma Square on the front steps of the assembly in central Athens on Wednesday.
AnalysisGathered outside Greece's parliament and making obscene gestures, members of the so-called Indignant movement chant "Kleftes" (or "thieves") at the country's politicians.
Their intention and that of the unions taking part in the general strike, is to force the government to throw out the new austerity programme.
Prime Minister George Papandreou has no intention of doing so. In a recent interview he compared Greece's plight to that of a country at war, saying mistakes are made, battles are lost and combatants are wounded, but he said he was determined to win.
The country is losing faith in his leadership, however. For the first time in years, his socialist party is lagging behind the opposition conservatives in opinion polls.
Mr Papandreou faces the risk of a backbench revolt over the plans.One MP defected from Mr Papandreou's PASOK party defected 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.
The government has appealed for consensus over its proposals, which would see 6.5bn euros (£5.7bn; $9.4bn) worth of 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 is demanding the measures in return for the release of another 12bn euros in aid next month which Athens needs to pay off maturing debt.