The head of Greece's main opposition party, Syriza on Friday took a temporary break from his duties to begin his mandatory military service.

Stefanos Kasselakis reported to the central town of Thiva for a 20-day period at a local army base.

Speaking to a crowd, he said it was "an honour" to serve in the town.

All adult Greek males must perform military service in the army, navy or airforce for a minimum period of nine months.

But Kasselakis, who has lived in the United States since his teens, is entitled to a reduced term that he can partly buy off.

Greek NBA basketball star Giannis Antetokounmpo made use of a similar provision when he served with his Milwaukee Bucks teammmate and brother Thanassis in 2016.

The 35-year-old former Goldman Sachs trader and businessman Kasselakis was in September a surprise winner to lead the leftist Syriza party.

It came after the party's longterm leader Alexis Tsipras, a former prime minister, resigned after a series of defeats, the latest in June national elections.

Kasselakis's pro-business policies swiftly alienated many Syriza leftists and the party suffered a damaging split earlier this year, with 11 lawmakers defecting to form a rival group.

He is also the first senior elected official in Greece to declare he is gay.