One of the attackers was killed when demonstrators fought back.
Rights campaigners accused police and ruling party officials in Democratic Republic of Congo on Tuesday of recruiting thugs to attack a peaceful opposition rally last month in which more than a dozen protesters were injured.
Tensions are high in Congo, where the opposition accuses President Joseph Kabila, who must step down at the end of his term next year according to the constitution, of manipulating a packed elections calendar in order to cling to power.
Young men with clubs and sticks stormed into the crowd at the Sept. 15 event in the capital Kinshasa, organised against Kabila's perceived attempts to extend his rule. One of the attackers was killed when demonstrators fought back.
Human Rights Watch (HRW) said around 100 men, paid some $65 each, were recruited from the youth wing of Kabila's People's Party for Reconstruction and Democracy (PPRD) and among supporters of local football team Vita Club.
HRW was present at the rally and spoke to witnesses, including several assailants.
"The apparent involvement of senior security and ruling party officials in the violent attack shows the ugly depths to which the authorities are willing to go to block opposition protests," said Ida Sawyer, senior Africa researcher at HRW.
Congo's Information Minister Lambert Mende denied state authorities or PPRD officials were involved in the attack and said HRW was simply repeating opposition accusations.
"We were surprised by the (HRW) communique," he said. "We condemned the violence."
Alleged participants in the attack told HRW they were called to a meeting at a military camp in Kinshasa the night before the rally.
"We were told to start attacking the demonstrators and create disorder as soon as one of the opposition leaders insulted President Kabila," one assailant told the rights group.
Witnesses alleged General Celestin Kanyama, Kinshasa's police commissioner, was among at least three senior officials to give instructions to the recruits.
Kanyama was not immediately available to comment.
Police providing security at the demonstration only intervened when the angry protesters turned on the assailants and started beating them, HRW said.
Kabila, 44, rose to power in Congo, Africa's largest copper producer, in 2001 after his father, President Laurent Kabila, was assassinated. He went on to win disputed elections in 2006 and 2011.
Mende has in the past said Kabila intends to respect the constitution, but the president has so far not commented publicly on his political future.