Umarali Kuvatov, the leader of Tajik opposition movement, the Group 24 was killed in Istanbul late on Thursday
Umarali Kuvatov was reportedly shot after a quarrel on a street in Fatih district, sources said.
The unidentified suspect fled the scene and the victim's body was transferred to a local morgue.
Officials said the case remains under investigation.
The Supreme Court of Tajikistan had labeled Group 24 an "extremist" group and had banned its activities in October 2014. The court decision came after the opposition group called online for anti-government protests.
Kuvatov was the AKP's alleged trump card in negotiations with Tajik government
Kuvatov's name was mentioned in the Gülen Community schools debate between Turkish and Tajik governments. Tajik President Emomalii Rahmon had allegedly announced that they could cancel the licenses of the six Gülenist schools in the country in exchange for extradition of Tajik opposition leader Kuvatov at the end of the last year.
Kuatov was the partner of President Rahmon's son-in-law Shamsullo Sahibov, in a company supplying oil and natural gas to NATO troops in Afghanistan before leaving the coun