SOFIA (Reuters) - Bulgaria's opposition GERB party boycotted parliament for a third day on Friday, blocking the work of the assembly because the ruling Socialists do not command the majority needed to open sessions.

The boycott, intended to demonstrate what GERB said was the ruling coalition's inability to govern, meant the opposition effectively blocked its own no-confidence motion against the government which was scheduled for Friday.

The standoff underlines a deepening political crisis which has deterred much-needed investment in the poorest EU country where the struggling economy is forecast to grow by 0.6 percent this year, compared with 0.8 percent in 2012.

It is likely to add to public disenchantment with the political elite which has seen thousands of people stage anti-corruption protests in recent months.

GERB's leader, former premier Boiko Borisov, demanded the resignation of Prime Minister Plamen Oresharski's government, which took office only four months ago.

"We need immediate elections. The government lacks a majority - on what basis do they want to rule?" he said.

The Socialists dismissed the boycott as irresponsible "circus tricks" which prevented the approval of laws aimed at fighting graft and poverty in the Balkan country. They said a government resignation and new elections were not options in times of economic hardship.

The party and their junior coalition partners, the ethnic Turkish MRF party, control 120 seats in the 240-seat parliament - one short of the 121 lawmakers required in the chamber to open sessions.

They enjoy the unofficial support of the nationalist Attack party, which has 23 seats. However Attack, whose anti-Roma and anti-Turkish policies have raised concerns among some EU officials, said its deputies were away in Brussels for meetings.

"The situation manifests the weakness of this government and will only make its key task, to survive at any cost, harder," said Daniel Smilov, a political analyst at the independent Centre for Liberal Studies.

The vote on the no-confidence motion is now set for next week.

Thousands of Bulgarians have protested against the government since it took office, including a siege of parliament in July. Rallies have eased recently, but opinion polls show about 50 percent of Bulgarians believe the government should step down by next May.

Borisov's centre-right GERB government resigned in February during protests over corruption and high utility bills.

(Reporting by Tsvetelia Tsolova; Editing by Pravin Char)