The Congolese Observatory for Human Rights (OCDH) on Saturday criticised as "lenient" jail terms of up to three years handed out to six police officers accused in the deaths of 13 young men.
The head of the Brazzaville police station where the 13 died last July was given a two-year prison sentence and his deputy a three years.
Three other policemen were jailed for one year, while a woman police officer, the only female in the group, was handed a 12-month suspended term.
They had been on trial since October on charges of manslaughter and criminal negligence towards a person at risk.
The campaigning body "expresses its deep indignation after the lenient verdict", said a statement following Friday's verdict from the court in the capital Brazzaville.
"This lenient verdict illustrates once again the trivialisation of torture and contempt for human life and confirms the difficulties torture victims face in seeking justice."
"The judgment is not equal to the blood crimes committed and will discourage torture victims from taking action," it said.
The group also voiced alarm that "no one in the hierarchy or in authority has been sanctioned, leaving people to think this drama is the sole responsibility of a few policemen acting outside established rules."
Around 20 youths had been picked up by police carrying out an "anti-delinquency" operation following a local murder.
The OCDH said the young men were "tortured and executed".
The Congolese state, as the employer of the police, admitted legal responsibility, but the court ruled it was not empowered to determine compensation.
That decision, which the OCDH said was "incomprehensible", left families grieving over the loss of a potential wage-earner.
The government paid out two million CFA francs (3,049 euros, $3,428) to each family towards funeral costs.