The parliamentary assembly of the Council of Europe on Tuesday asked the US government "to cease torturing and mistreating detainees" at its facility in Guantanamo Bay, Cuba.
"The US government has betrayed its own highest principles in the zeal with which it has attempted to purse the war on terror," it said.
"These errors have perhaps been most manifest in relation to Guantanamo Bay," said the resolution passed by parliamentarians of the pan-European body at a session in Strasbourg.
Based on reports by legal and human rights organisations, many if not all detainees have been subjected to inhuman treatment, in some cases amounting to torture, the resolution said.
This has occurred as a direct result of official policy, authorised at the very highest levels of government, the resolution added.
The US began using its naval base at Guantanamo Bay as a detention centre for "war on terror" suspects after the invasion of Afghanistan in 2001.
"The US government has betrayed its own highest principles in the zeal with which it has attempted to purse the war on terror"
Council of Europe resolution
US forces ousted the Taliban government, an ally of the al-Qaida group which claimed responsibility for the 11 September attacks that killed nearly 3000 people in the United States.
The assembly said it supported the US efforts to fight terrorism, but that they had to be conducted in line with international law.
The US conduct in Guantanamo has been "an insult to the values of all civilised countries. We must respond firmly to threats but not in an unjust manner," said British parliamentarian Kevin McNamara at the session.
The assembly plans to pursue the issue through talks with the US Congress. The United States has observer status at the 46-member Council of Europe.