ANKARA, Turkey—An official says Turkish authorities have tightened security for the Istanbul-based Orthodox Patriarchate following allegations of a plot to attack the spiritual leader of the world's Orthodox Christians.

Unconfirmed local media reports said Friday police briefly detained a man following a tip that he planned to assassinate Ecumenical Patriarch Bartholomew I.

Haberturk newspaper said prosecutors in Ankara were investigating the alleged plot. Police wouldn't confirm the reports, and the prosecutor's office did not immediately return calls.

Rev. Dositheos Anagnostopoulos, the spokesman for the Patriarchate, said he could not confirm the alleged plot, but said Turkish authorities had increased protection around the church Friday.

The Patriarchate is based in Istanbul—formerly Constantinople—which was the capital of Christian Byzantium before falling to Muslim forces in 1453.