Portugal's Communist Party on Friday said it had formed an alliance with the main opposition Socialists, putting the leftists in a position to bring down the country's new centre-right government.

Together, the Socialists, Communists and their allies hold 122 seats out of 230 in Portugal's parliament -- more than half the votes needed for a motion of no confidence in the new government to succeed next week.

"The conditions are in place to bring down the right-wing coalition government and for the Socialist Party to form a government, present its programme and take office," the Portuguese Communist Party said in a statement.

The centre-right coalition of Prime Minister Pedro Passos Coelho won an October 4 general election, but it lost the outright majority it had enjoyed since 2011.

The Communists' statement came hours after the Left Bloc, allied to Greece's governing anti-austerity Syriza party, agreed its own pact with the Socialists (PS).

"Negotiations with the Socialist Party have ended and the conditions (are) met for a leftist agreement aiming to protect jobs, salaries and pensions," a statement on the Left Bloc's website said.

The Socialists, who came second in the election, are seeking to oust the government in tandem with other left-wing parties in order to end Portugal's austerity drive.

The PS has called a meeting for late Sunday to assess the negotiations, ahead of a parliamentary debate on the government's programme the following day. If the leftists vote against the programme the government would be compelled to resign.