Oyub Titiyev, head of the Chechnya branch of rights group Memorial, was arrested at the start of last year on what supporters said were trumped-up charges.

The 61-year-old was in court in Chechnya's Shali district for the marathon sentencing, which exceeded nine hours, images and statements from Memorial showed.

Memorial is the last rights organisation to retain a visible presence in Chechnya while remaining critical of local leader Ramzan Kadyrov.

Titiyev's lawyers said the case was connected to his investigations into the secretive prison system of a republic that is notorious for rights abuses.

Prosecutors had called for a sentence of four years and a fine of 100,000 rubles ($1,500, 1,350 euros).

The trial has been ongoing since last summer.

Observers say drug charges have been used repeatedly in the past to silence critical journalists and rights workers in the region.

Amnesty International said the verdict demonstrated how "deeply flawed" the Russian justice system is.

"The four-year prison sentence slapped on Oyub Tititev is an affront to human rights, reason, and justice," it said in a statement.

"The court has revealed itself to be little more than a tool that the regional authorities have used to silence one of the last human rights defenders working in Chechnya."

Earlier Amnesty had called for Titiev's unconditional release, saying he was a prisoner of conscience.

'Unfair and absurd'

The Paris-based International Federation for Human Rights also condemned the verdict, saying it followed an "unfair and absurd" judicial process.

Memorial said its offices in the neighbouring Ingushetia republic were torched following Titiyev's arrest and colleagues in Dagestan, another Caucasus republic, were attacked.

The group speaks out about human rights violations in Russia and has accused the Chechen leader of overseeing a "totalitarian" regime that uses kidnappings and torture.

Titiyev's predecessor as the regional leader of Memorial, Natalya Estemirova, was kidnapped and killed in 2009. The rights group said the killers were acting on the orders of local authorities.

The verdict came a week after the Council of Europe committee rebuked Russia for failing to halt the police torture of detainees in Chechnya.

A delegation of the council's anti-torture committee said that torture of detainees in Chechnya remained "a deep-rooted problem".

A Russian LGBT rights group in January reported a new wave of persecution against gay people in Chechnya.

Local authorities immediately denied the claims which came two years after an international outcry when gay men said they had been tortured by law-enforcement agencies in the majority-Muslim republic.