According to Turkish mass media reports, an unknown individual came close to the fugitive businessman and shot him in the head.
Kuvvatov, who had been living in exile in Turkey and was head of the Group 24 opposition movement, was killed with a single shot to the head at around 10:30 pm on Thursday evening in the Fatih district of the city, Dogan news agency said. The attacker then fled the scene.
The 47-year-old businessman was already dead when medics arrived at the scene and police searched the area for evidence, it said.
The oppositionist fled from Tajikistan in 2012. He created the organisation Group of 24 which was recognised extremist by the Tajik Supreme Court. He was an outspoken critic of Tajik leader Emomali Rakhmon.