Security forces killed at least 24 people and "arbitrarily" arrested dozens more in the Democratic Republic of Congo since Joseph Kabila's contested re-election on December 9, Human Rights Watch said.
"At least 24 people were killed by security forces between December 9 and 14, including 20 in Kinshasa, two in (eastern) North Kivu, and two in (central) Kasai Occidental province," HRW said in a statement.
"Human Rights Watch also documented an incident in which local youth in Kinshasa threw rocks at a priest who later died from his injuries," it added.
Since the election commission issued results on December 9 showing Kabila had won the November 28 presidential vote, "security forces have been firing on small crowds, apparently trying to prevent protests against the result," said Anneke Van Woudenberg, senior Africa researcher at Human Rights Watch.
"These bloody tactics further undermine the electoral process and leave the impression that the government will do whatever it takes to stay in power," she said.
The human rights watchdog said after interviewing 86 victims and witnesses it had received "dozens of reports of other killings and attacks by security forces which it is seeking to confirm and is continuing its investigations."
Kabila's victory with 48.95 percent of the vote was upheld even after international observers criticised electoral conditions, citing problems in the vote count and the loss of huge numbers of ballots.
Opposition leader Etienne Tshisekedi, given 32.33 percent, contends that he won the poll but was denied victory by massive fraud.
He issued thousands of leaflets Wednesday calling on people to attend his own "swearing-in" ceremony taking place three days after re-elected head of state Kabila took office.
HRW said in its statement that "police and other security forces appear to be covering up the scale of the killings by quickly removing the bodies." It singled out the police and the republic guard -- the presidential bodyguard -- for blame.
The statement went on: "Several sources informed Human Rights Watch that the government had instructed hospitals and morgues not to provide information about the number of dead or any details about individuals with bullet wounds to family members, human rights groups, or United Nations personnel, among others.
"The UN and Congo's international partners should urgently demand that the government rein in its security forces."
Tshisekedi, 79, is challenging the outcome of the vote which the country's supreme court and the election commission said Kabila had won by a large majority.
Since Sunday, the presidential guard has positioned tanks outside the stadium and throughout the capital. Police have regularly moved to break up gatherings of opposition groups.
London-based rights group Amnesty International on Wednesday denounced what it said was a wave of political arrests, notably of opposition activists, since the elections.
The opposition leader plans to hold his "inauguration" ceremony at Kinshasa's Martyrs Stadium on Friday at 0900 GMT, a move that could trigger more violent clashes in the vast nation.
