Ben Emmerson, the UN’s special rapporteur on counter-terrorism and human rights, said all CIA and other US government officials who carried out waterboarding and other torture must also be prosecuted.
A US Senate Intelligence Committee report detailed use of the techniques as part of former President George W Bush’s national security policies.
In a statement released in Geneva, Mr Emmerson said the report shows “there was a clear policy orchestrated at a high level within the Bush administration, which allowed (it) to commit systematic crimes and gross violations of international human rights law”.
He said those responsible for the “criminal conspiracy ... must face criminal penalties commensurate with the gravity of their crimes”.
Senate investigators concluded that the US brutalised scores of terror suspects with interrogation tactics that turned secret CIA prisons into chambers of suffering and did nothing to make America safer after the September 11 attacks.
The report accused the CIA of misleading its political masters about what it was doing with its “black site” captives and deceiving Americans about the effectiveness of its techniques.
The report, the first public accounting of tactics employed after 9/11, described far harsher actions than had been widely known.
Tactics included confinement to small boxes, weeks of sleep deprivation, simulated drowning, slapping and slamming, and threats to kill, harm or sexually abuse families of the captives.
President Barack Obama told the Spanish-language TV network Telemundo that some of the past practices were “brutal, and as I’ve said before, constituted torture in my mind. And that’s not who we are.”
He added: “One of the things that sets us apart from other countries is that when we make mistakes, we admit them.”
Then-president Mr Bush approved the programme through a covert finding in 2002, but he was not briefed by the CIA about the details until 2006.
At that time he expressed discomfort with the “image of a detainee, chained to the ceiling, clothed in a nappy and forced to go to the bathroom on himself”.
Five hundred pages were released, representing the executive summary and conclusions of a still-classified 6,700-page full investigation.
Senator Dianne Feinstein, the Democratic committee chairman whose staff prepared the summary, branded the findings a stain on US history.
“Under any common meaning of the term, CIA detainees were tortured,” she declared.
In a statement, CIA director John Brennan said the agency made mistakes and has learned from them.
But he also asserted that the coercive techniques “did produce intelligence that helped thwart attack plans, capture terrorists and save lives”.
Former Polish president Aleksander Kwasniewski said that during his term Poland offered the CIA a site for a secret prison but did not authorise the harsh treatment of inmates.
It is the first time a Polish leader has admitted the country hosted a secret CIA site. Reports say it operated from December 2002 until the autumn of 2003.
Mr Kwasniewski was in power from 1995 to 2005. He said the activity in Poland was terminated under pressure from its leaders.