The Working Group on Arbitrary Detention report particularly criticised the "imprecise definitions of crimes" in anti-terror legislation and the use of military tribunals and special courts of law.
The UN group's criticisms come as the United States finds itself under increasing fire for its extrajudicial procedures for detainees held at its Guantanamo military base in Cuba.
But Washington has rejected the UN report, saying the working group is not competent to judge the matter.
"States do not have the right to controvert principles as fundamental as the presumption of innocence ... and the right to be judged within a reasonable period of time by a competent, independent and impartial tribunal," the report read.
Since the 11 September 2001 attacks in the US, a significant number of people from several countries have been arbitrarily detained as a result of new anti-terror laws, the UN experts continued.
Some have been held in secret locations set up for presumed terrorists, and others - notably those at Guantanamo - have been held without trial and deprived of the rights due to either indicted criminals or prisoners of war.
The UN experts said Washington did not have the right to detain indefinitely the hundreds of detainees at Guantanamo - mostly men captured during US military operations against the Taliban and al-Qaida in Afghanistan.
"States do not have the right to controvert principles as fundamental as the presumption of innocence"
Working Group on Arbitrary
"In no case may an arrest - made under laws of exception - be prolonged indefinitely," they said in the report.
Last week US Defence Secretary Donald Rumsfeld defended the indefinite detention without trial or charges of the Guantanamo men.
He said their cases would be reviewed on an annual basis and that those deemed no longer a threat would be released.
Of the 660 detainees in the Cuba base since October 2001, less than 100 have been released.
Three boys aged between 13 and 15 were released from Guantanamo late in January, a year after having been captured by US forces in Afghanistan.
A Spanish Muslim was also released last week after lengthy negotiations between Madrid and Washington, and other countries including Britain are in talks to gain custody of their nationals held there.
The UN experts also denounced the formation of lists of suspected terrorists who are named without any proof.
The Working Group on Arbitrary Detention is comprised of five independent experts. The preliminary report released on Monday will be debated by the UN Commission on Human Rights on 15 March.