Afghan authorities should investigate the arrests and possible torture of peaceful protesters by security forces in Kabul, Human Rights Watch said.
The abuses appear intended to silence public dissent against the government.
On May 2, hundreds of people participated in a demonstration in Kabul's Cinema Pamir neighbourhood organised by the Solidarity Party of Afghanistan to protest the government's failure to prosecute abusive warlords, including those now in official positions.
Security forces cut short the protest and arrested at least nine people. Six of them told Human Rights Watch they were mistreated in custody for up to three days.
"The arrest and brutal beating of peaceful protesters seems aimed at sending a message to all Afghans not to publicly criticize the government," said Brad Adams, Asia director at Human Rights Watch.
"President (Hamid) Karzai should order an immediate investigation of the security forces' actions and appropriately punish all those responsible for these abuses."