* Kuvvatov accused veteran Tajik leader of corruption

* Kuvvatov was little-known in his own country - analyst

* Turkey had declined Tajik request to extradite him (Adds analyst comment, background)

By Daren Butler and Dmitry Solovyov

ISTANBUL/ALMATY, March 6 (Reuters) - An outspoken critic of veteran Tajik leader Imomali Rakhmon has been shot dead by an unknown assailant on a street in Istanbul, Turkish media reported on Friday.

Umarali 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 (2030 GMT) on Thursday evening in the city's Fatih district, Dogan news agency said. Istanbul police declined to comment on the case.

Turkey's anti-terrorism police unit and murder squad are handling the investigation into the killing of the 47-year-old businessman, Dogan said.

Kuvvatov had accused President Rakhmon, 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.

Kuvvatov had worked for a business of one of Rakhmon's close relatives. He took to politics after he fell out with Rakhmon's clan due to an apparent clash of business interests, said Alexander Knyazev, a Kazakhstan-based Central Asia analyst.

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

The paper quoted people close to Kuvvatov 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.

A spokesman for the Tajik prosecutor-general's office said it had information about the killing but gave no details.

Last October, Kuvvatov appealed via social networks for an open-ended protest in the Tajik capital Dushanbe to topple Rakhmon and his government. But people did not show up and the authorities increased the police presence in the city and briefly blocked some social media.

Kuvvatov's "Group 24" movement was then declared an "extremist organisation" and banned by Tajikistan's Supreme Court.

"No one came to support him at the time. It's not just about official pressure - people simply don't know him, and he has no sensible or coherent political programme," said analyst Knyazev.

Tajikistan wanted him for a number of crimes, including extremism, economic crimes and hostage-taking.

Kuvvatov had lived in Russia and the United Arab Emirates before moving to Turkey. Turkey had refused a Tajik request to extradite him. (Additional reporting by Dmitry Solovyov in Almaty; Writing by Daren Butler; Editing by Nick Tattersall and Gareth Jones)