Human Rights Watch (HRW) has accused the government of the DR Congo of using violence to eliminate political opponents since polls in 2006.
According to the rights group, Congolese security forces had deliberately killed more than 500 people in a campaign against opposition groups. (photo, from bbc.co.uk)
A government spokesman told the BBC the allegations were false.
As everyone was focusing on the conflict in the east, the abuses were attracting scant attention, said HRW.
Since August, when fighting erupted between government troops and rebels loyal to Tutsi General Laurent Nkunda, more than 250 000 people have fled their homes.
UN Secretary General Ban Ki-moon said on Monday that all parties involved in the conflict had committed serious human rights abuses.
Congolese president Joseph Kabila has been urged by UN special envoy, Olusegun Obasanjo, to talk with Gen Nkunda to prevent the situation from worsening.
The elections in July 2006 aimed at bringing democracy to the country after years of fighting. But in HRW’s reports, Mr Kabila’s government is accused of “brutal repression” following the elections.
Since then, five hundred perceived opponents have been killed, and another 1 000 have been detained, many of whom reported being tortured, said the rights group.
Many of those targeted were supporters of defeated presidential candidate Jean-Pierre Bemba and of another political group in the west of the country.
According to HRW opposition groups have also used violence. “In these cases, the police and army had a duty to restore order, but often did so with excessive force”, it said.
The report did not reflect the facts, said Lambert Mende, a government spokesman.
He added that Congolese judges could not avoid convicting people who had committed crimes on the basis that they were political opponents.
However a senior researcher for HRW, Anneke Van Woudenberg, said the group had documented the use of “brutal force” against government critics.
“This, of course, together with the rebellion in the east, is dramatically undermining Congo’s ability to develop a democratic state”, she told the BBC.
An additional 3 000 soldiers and police for the UN peacekeeping mission in DR Congo has been approved last week by the UN Security Council, in an attempt to prevent the conflict in the east from escalating.
Tens of thousands of people have been displaced by the fighting in North Kivu province, and aid agencies are struggling to reach them.
Arbitrary executions, mass killings, rape and torture have been carried out by both government forces and rebel troops there, said the UN leader on Monday.
In a 28-page report for the UN Security Council, Ban Ki-moon said the human rights situation there was a “cause for grave concern”.
The UN said that special envoy Olusegun Obasanjo is due to return to DR Congo at the weekend, for more talks aimed at ending the conflict.