A hand grenade shattered church and monastery windows at the Greek Orthodox patriarchate in the Turkish city of Istanbul early on Thursday but no one was hurt, officials said.
"Two windows in the church and five in the monastery were smashed in the explosion," a patriarchate official told Reuters.
It was not immediately clear who was behind the attack, which took place at around 1:30 a.m. (2230 GMT).
Nationalists in overwhelmingly Muslim Turkey have targeted the patriarchate in the past.
Last month, far-right protesters clashed with police outside the complex and burned an effigy of Patriarch Bartholomew, spiritual leader of the world's 140 million Orthodox Christians. They oppose allowing the Greek Orthodox Church to own land.
Istanbul, then known as Constantinople, was the centre of Orthodox Christianity until it fell to the Muslim Ottoman Turks in 1453. The patriarch remains nominal head of the church, though the number of Orthodox Christians in Turkey is now tiny.
Turkey does not recognise Bartholomew as the ecumenical leader of Orthodox Christians.
The European Union, which Turkey aspires to join, has criticised state curbs on the Greek Orthodox Church and other non-Muslim communities.
The European Commission recommended on Wednesday that the 25-nation bloc begin entry talks with Turkey but pointed to areas where human rights must be improved, including restrictions on the patriarchate's legal and property rights.