Turkish Muslim extremist Metin Kaplan, alias "The Caliph of Cologne", told a tribunal in Istanbul on Monday how he tried to convert the late Pope John Paul II to Islam. "When I was in Germany, before the pope's death, I wrote him a letter, asking him to convert to Islam," Kaplan told the court. The prosecutor in the case against Kaplan sought life imprisonment for him on Monday, accusing him of trying to forcefully overthrow Turkey's secular political system. Among the charges against Kaplan, leader of the Cologne-based extremist group known as the Caliphate State -- is an alleged 1988 bid to use an explosives-laden plane to blow up the mausoleum in Ankara of Mustafa Kemal Ataturk. Lawyers for Kaplan, on the second day of his trial that opened in Turkey in December, denied there was a bid to bomb the mausoleum, and called on the court to act in accordance with judicial practice in the European Union, which Turkey is seeking to join, and free his client on parole. The court threw out the request and adjourned the trial to May 30.