An outspoken critic of Tajikistan’s President Emomali Rahmon has been shot dead by an unknown assailant on a street in Istanbul, Turkish media has reported.

Umarali Kuvatov, 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 in the Fatih district of the city, Dogan news agency said. The attacker then fled the scene.

Turkey’s anti-terrorism police unit and murder squad are handling the investigation into the killing of Kuvatov, who had been eating dinner at a house in the area before he was shot, Dogan said.

The 47-year-old businessman was already dead when medics arrived at the scene and police searched the area for evidence, it said.

Kuvatov had accused Rahmon, a 62-year-old former head of a Soviet state farm who has governed the impoverished central Asian republic since 1992, of rampant corruption and nepotism.

A spokesman for the Tajik prosecutor-general’s office said it had information about the killing but gave no details. He said Tajik prosecutors had not yet been in contact with their Turkish counterparts about the shooting.

Turkey’s Sabah newspaper said Kuvatov and his family had fallen ill after eating dinner with a fellow Tajik, who has since been detained by police. It said Kuvatov had left his house to take his wife and children to hospital.

The paper quoted people close to Kuvatov as saying he and his family may have been poisoned before he was shot. It said the assailant was believed to be Tajik and had said a few words before opening fire.

Kuvatov’s Group 24 movement was declared an extremist organisation and banned by Tajikistan’s supreme court last October. Tajik law enforcers wanted him for a number of alleged crimes, including extremism, economic crimes and hostage-taking. Turkey had declined to extradite him.

Kuvatov had worked for a company trading oil products that was headed by one of Rahmon’s relatives. He founded his movement in 2012 after emigrating to Russia.

When Tajikistan asked Moscow to extradite him, he moved to the United Arab Emirates where he was detained at the request of Tajik police. He then settled in Turkey.