Human Rights Watch said it concluded "the US-administered system of arrest and detention in Afghanistan exists outside of the rule of law".
The United States has detention facilities at the Bagram, Kandahar, Jalalabad and Asadabad military bases.
"The United States is setting a terrible example in Afghanistan on detention practices," said Brad Adams, executive director of the Asia division of Human Rights Watch, on Sunday.
"Civilians are being held in a legal black hole - with no tribunals, no legal counsel, no family visits and no basic legal protections."
And Washington has not responded adequately to questions about arrest and detention practices, the group charged.
It gave the example of three detainees it said were known to have died while in US custody, two at the Bagram airbase north of Kabul in December 2002 and one at Asadabad in June 2003.
"The United States is setting a terrible example in Afghanistan on detention practices. Civilians are being held in a legal black hole - with no tribunals, no legal counsel, no family visits and no basic legal protections"
Human Rights Watch
The first two deaths were ruled homicides by US military pathologists who performed autopsies on the two men, but US officials have yet to explain what happened to any of the three men, the watchdog said.
"This stonewalling must stop," said Adams. "The United States is obligated to investigate allegations and prosecute those who have violated the law. There is no sign that serious investigations are taking place."
The report is based on research conducted by Human Rights Watch in Afghanistan and Pakistan in 2003 and early 2004.
"Human Rights Watch documented cases of US forces using military tactics, including unprovoked deadly force, during operations to apprehend civilians in uncontested residential areas and in situations where law enforcement standards and tactics should have been used," the group said.
"Released detainees have said that US forces severely beat them, doused them with cold water and subjected them to freezing temperatures.
"Many said they were forced to stay awake, or to stand or kneel in painful positions for extended periods of time," according to the report.
It also reported that "frequent arbitrary arrests of civilians, apparently based on mistaken or faulty intelligence" have been carried out.
US President George Bush and officials in his administration said in June 2003 the US did not torture or mistreat detainees in its custody.
"But the United States has refused to allow any independent observers access to detention facilities in Afghanistan, except for the International Committee of the Red Cross, which does not report publicly on its findings," the rights group said.
Human Rights Watch noted that some documented abuses in the report took place after President Bush's statement.