"Our sources were very credible and the evidence was corroborated by all the different groups we spoke to," said the MEP. "They left me in no doubt of the veracity of the claims."

But Turkey's foreign ministry spokesman, Namik Tan, called the claims "silly stories". "They are purely fictitious. They have nothing to do with the truth. You won't find anyone who is credible in Turkey saying such things."

Mr Howitt said that in September alone 95 people had been arbitrarily arrested in Van, a town near Iran. Among them was Yusuf Hasar, a 19-year-old suspected Kurdish rebel sympathiser whose body was found last week after being arrested by police the previous day. The violations have coincided with an upsurge of violence in Turkey's troubled south-east. Armed clashes have intensified since rebels lifted a unilateral ceasefire in June last year.

The delegation, whose findings will form the basis of a report that will feed into Turkey's membership negotiations, was equally appalled by reports of violence against women and allegations of body organs being removed by security forces. Mazumber, a group representing the relatives of torture victims, told the MEPs that vital organs were routinely removed from the bodies of ethnic Kurds, presumably as part of the illicit trade in people trafficking.

Mr Howitt said it was essential the abuses be confronted before Ankara got into the nitty-gritty of the talks.

Since assuming power in 2002, Ankara's modernising Islamist government has won plaudits for overhauling the penal code, abolishing the death penalty, dismantling once-dreaded state security prisons and increasing cultural rights for ethnic minorities. But Turkish human rights defenders still speak of a pervasive "culture of violence" in the country's police, security and judicial forces.