ATHENS, Jan. 5 (Xinhua) -- Former Greek Prime Minister Costas Simitis passed away at the age of 88 on Sunday at his country house near Athens, Greek national news agency AMNA reported.

The government has declared four days of national mourning to honor the leader who played a pivotal role in Greece's entry into the eurozone in 2001.

Simitis served as prime minister from 1996 to 2004, championing economic and social reforms while advancing Greece's integration into the European Union.

"We bid farewell to a competent academic teacher and a moderate figure in parliament. But above all, a catalyst of public life, who boldly put the great effort of modernizing the country at the center of the focus," Greek Prime Minister Kyriakos Mitsotakis said in a statement.

Simitis was a respected political rival, who left his imprint on the country's development over the past decades, he added.

Born in Piraeus, Simitis studied Law and Economics in Britain and Germany, where he worked for years as a university professor. Simitis was a founding member of the Panhellenic Socialist Movement (PASOK) in the 1970s, a party that became a dominant force in Greek politics for decades and which he eventually led. ■