The rights violations appear designed to suppress dissent against the government, Human Rights Watch said Tuesday.
State security forces arrested at least nine people of the hundreds at a May 2 demonstration in Kabul organized by the Solidarity Party of Afghanistan.
Those arrested told Human Rights Watch they had been mistreated while in custody for three days. They alleged they were punched, kicked and beaten with rifle butts.
Two of those arrested were seriously injured and several detainees said their personal property was taken. All were released without charges.
Brad Adams, Asia director for Human Rights Watch, said the abusive treatment "seems aimed at sending a message to all Afghans not to publicly criticize the government. President (Hamid) Karzai should order an immediate investigation of the security forces' actions and appropriately punish all those responsible for these abuses."
The Afghan interior ministry had granted a permit for the demonstration, Human Rights Watch said, but security forces encouraged residents and business people to leave the area. Several hundred protesters were denied access to the area.
The Interior Ministry hasn't responded to requests for information about the actions of the security forces, Human Rights Watch said.