“Over the past three years, 14,261 people have been unlawfully prosecuted,” Prosecutor-General Yury Chaika told legislators, the state-run RIA Novosti news agency reported.

“People are spending years in jail without any legal basis,” Chaika said.

Chaika added that more than 4,600 people had been illegally detained and held in custody over the same period.

Russian courts had an acquittal rate of around 4.5 percent in 2013, the chairman Supreme Court chairman Vyacheslav Lebedev told journalists this week.

Jury trials, which remain rare, had a much higher acquittal rate of around 20 percent.

Human rights activists regularly criticise Russia’s justice system, alleging rampant corruption and politically motivated prosecutions of those critical of the authorities.

In December a Kremlin-backed general amnesty saw thousands of inmates released from jail including former oil magnate Mikhail Khodorkovsky and two members of the punk protest group Pussy Riot, but rights activists called this a token move.

The freed band members– Nadezhda Tolokonnikova and Maria Alyokhina– have pledged to fight for better prison conditions and have talked publicly of their experiences of abuses in jail.