European governments were almost certainly aware of the CIA's secret prisoner flights via European airspace or airports, a key report has said.
Dick Marty says his investigation into the claims is continuing
The preliminary report comes from Swiss MP Dick Marty, for the human rights watchdog the Council of Europe.
The US admits picking up terrorism suspects but denies sending them to Arab nations to face torture.
Mr Marty said he could not be certain that the CIA used secret prisons in Europe to interrogate terror suspects.
The BBC's Tim Franks at the Council of Europe says the Swiss senator's report does not appear to reveal hard new facts.
'Great deal of evidence'
Mr Marty began his investigation in November and presented his interim report to the Council of Europe in Strasbourg on Tuesday morning.
In his report, he said "rendition" - the secret transport of prisoners via Europe to third countries where they may have been tortured - seemed to have affected more than 100 people in recent years. The facts had "not even been denied", he said.
At a news conference he denounced the practice as "criminal acts" which "run counter to the laws that prevail in all civilised countries today".
He cited the case of the Egyptian political refugee, Abu Omar, who was, he said, abducted by the CIA in the middle of the Italian city, Milan, in 2003, flown to Egypt and then tortured.
The report said his case was just one part of "a great deal of coherent, convergent evidence pointing to the existence of a system of 'relocation' or 'outsourcing' of torture".
Mr Marty singled out the Italian judiciary for praise and said the CIA kidnapping had "completely destroyed" an Italian police investigation into Abu Omar and his associates.
In another case, a German man was said to have been kidnapped in Macedonia and taken to Afghanistan.
COUNCIL OF EUROPE
Founded in 1949 and based in Strasbourg, France
Forty-six members, 21 of them from Central and Eastern Europe
Set up to defend human rights, parliamentary democracy and the rule of law
Acts as human rights watchdog for Europe
Oversees the European Court of Human Rights
Comprises a decision-making committee of ministers and 630-member parliamentary assembly
Mr Marty also looked into allegations of secret CIA detention centres in Romania and Poland. Here he said there was "no formal, irrefutable evidence", although he said that there should be further investigation.
Several Eastern and Central European countries accused of hosting them have all strenuously denied the charge.
Mr Marty's report talks about CIA activities challenging the very functioning of the law-based state and its democratic foundation.
"Acts of torture or severe violation of detainees' dignity through the administration of inhuman or degrading treatment are carried out outside national territory, and beyond the authority of national intelligence services...
"It is highly unlikely that European governments, or at least their intelligence services, were unaware," the report said.
Mr Marty told the Council of Europe that the investigation into CIA renditions was continuing.
He welcomed the fact that the European Parliament had also set up an ad hoc committee to investigate the controversial flights.
And he said he had received detailed information on Monday from Eurocontrol - the Brussels-based air safety organisation - and from the EU's Satellite Centre, including sites located in Romania.
The prisons story broke in early November, when the Washington Post newspaper said the CIA had been running facilities in Eastern Europe, Afghanistan and Thailand.
It said more than 100 people had been sent to facilities known as "black sites" since they were set up following the 11 September 2001 attacks.