Greece will elect its first woman president on January 22 after leading opposition parties on Thursday approved the government's nomination of top judge Ekaterini Sakellaropoulou.

Main opposition leftist leader Alexis Tsipras said Sakellaropoulou was an "exceptional judge" and a defender of human rights.

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The socialist KINAL party also said it would support the nomination of the 63-year-old current head of the Council of State, Greece's top administrative court.

Prime Minister Kyriakos Mitsotakis had announced Sakellaropoulou's candidacy in a surprise televised address on Wednesday.

"The time has come for Greece to open up to the future," Mitsotakis said, emphasising that the selection breaks with tradition not only because Sakellaropoulou is female, but also because she is not a member of a political party.

He said the choice "embodies unity and progress".

Two hundred votes are required to elect the new president for a five-year term.

Sakellaropoulou, a French-educated expert in constitutional and environmental law, is on track to secure more than 260 votes.

The term of incumbent President Prokopis Pavlopoulos ends in March.

The president is nominally the head of the Greek state and the post is largely ceremonial.