ATHENS (Reuters) - Greece's main opposition party, Syriza, said on Thursday it would back the conservative government's nominee for the presidency, paving the way for Katrina Sakellaropoulou to become the country's first female head of state.

Sakellaropoulou, 64, is now president of the Council of State, Greece's top administrative court. She will succeed President Prokopis Pavlopoulos, whose term expires in March.

The process to select a president, a largely ceremonial role, can go up to five voting rounds in parliament, with the election threshold starting at 200 votes and gradually falling to the majority of those present in the room.

Prime Minister Kyriakos Mitsotakis' New Democracy party controls 158 seats in the 300-seat house.

To win straight away, his presidential candidate would need support from opposition parties including leftist Syriza, which lost power in an election last July. Syriza has 86 deputies in parliament.

"We will allow the new president to be elected with a wide majority, as this is appropriate for anyone who takes on the heavy responsibility, under the constitution, of expressing the unity of the people and our democracy," Syriza Leader Alexis Tsipras said in a televised statement.

The Socialist PASOK party, with 22 deputies, convened to discuss the issue on Thursday but it was also expected to support Sakellaropoulou.

(Reporting by Renee Maltezou, Editing by Timothy Heritage)