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.

Activists and unionists plan to gather at Syntagma Square on the front steps of the assembly in central Athens on June 15.

Mr Papandreou faces the risk of a backbench revolt over the plans.

One MP defected from Mr Papandreou's PASOK party defected on June 14, leaving it with only 155 of the chamber's 300 seats.

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 US$9.4bn worth of tax rises and spending cuts this year.