Kemal Kilicdaroglu, leader of the Republican People’s Party (CHP), made the remarks at Chatham House, a London-based think tank, on the final day of his visit to the British capital.
He said: "Minorities living in Turkey can elect their own religious leaders according to the Lausanne Treaty. But Greece does not comply with this rule,” he said, referring to the landmark 1923 treaty.
The election of religious leaders or muftis has been a chronic problem of Greece’s Muslim Turkish minority -- concentrated in the Western Thrace region -- since 1991.
The election of muftis by Muslims in Greece was covered by the 1913 Treaty of Athens between Greece and the Ottoman Empire and was later included in the Greek Act 2345/1920, near the dawn of the Turkish Republic.
The 1923 Treaty of Lausanne also guarantees the religious freedom of the Muslim minority in Greece.
However, Greece annulled the Greek Act in 1991 and started appointing the muftis itself.
The majority of Muslim Turks in the Western Thracian cities of Komotini (Gumulcine) and Xanthi do not recognize the appointed muftis and instead elect their own muftis, but the latter are not recognized by the Greek state.
Despite this situation, the appointed muftis in Western Thrace continue to have authority to adjudicate family and inheritance matters of local Muslims.
Turkey has long complained to both Greece and the EU of discrimination against the Turkish minority concentrated in Western Thrace, saying this violates applicable treaties and EU law.
Kilicdaroglu said he paid a visit to the elected Mufti instead of the state-appointed one when he visited Greece.
"This is my democratic nature," he said, adding that Greece must comply with the decision by the European Court of Human Rights (ECHR) in this regard.
Turkey’s President Recep Tayyip Erdogan said Friday that Athens must implement European human rights rulings upholding the rights of the Muslim-Turkish minority in Greece’s Western Thrace region.
Kilicdaroglu met British Labour Party leader Jeremy Corbyn, and representatives of the Turkish community living in the U.K. during his visit to London. The CHP leader will return to Turkey on Saturday.