DR Congo police fired tear gas at supporters of opposition leader Etienne Tshisekedi and banned his "swearing-in" ceremony on Friday, saying President Joseph Kabila was the legally elected head of state.
Police used tear gas to disperse opposition supporters who had gathered near Tshisekedi's residence in Kinshasa's Liemete district and made several arrests, an AFP journalist said.
Armoured vehicles of the Republican Guard and a large number of police had also taken up positions around the capital's main Martyrs Stadium where Tshisekedi had called on his supporters to attend his swearing-in.
They also used tear gas at the stadium.
"It's banned. There is already an elected president who has been sworn in. We cannot have another swearing-in. It's an act of subversion," a source close to the head of the country's police said.
"Such a rally would be destabilising for the regime in place," he added.
Kabila, who has been in power since January 2001, took nearly 49 percent of the vote in last month's election, with Tshisekedi coming in second with 32 percent.
Kabila was officially sworn-in at a ceremony in Kinshasa on Tuesday.
Tshisekedi, 79, contends he won the poll but was denied victory by massive fraud.
His supporters on Friday repeated that argument.
"All the Congolese people voted for him. Why do they want to kill the Congolese people? They should let our president go to the stadium and let us accompany him," a supporter told AFP.
"Look they have deployed armoured cars. For what? These should be sent to the east of the country where there is war," he said.
The government Thursday pledged to probe alleged post-vote police killings, as activist group Human Rights Watch said security forces killed at least 24 people and "arbitrarily" arrested dozens since Kabila's disputed victory was announced December 9.
Justice Minister Emmanuel Luzolo Bambi told AFP his office would work with Human Rights Watch to try to document each case in the report, and that he had already spoken with prosecutors.
"If the allegations are verified, the justice department will take action," he said.
According to Human Rights Watch, all but four of those in its report died in Kinshasa between December 9 and 14. Two more were killed in eastern Nord Kivu province, and two in central Kasai Occidental.
HRW said it also documented an attack where youths in the capital stoned a priest, who later died of his injuries.
Since Kabila's victory was announced, "security forces have been firing on small crowds, apparently trying to prevent protests against the result," HRW senior Africa researcher Anneke Van Woudenberg said.
After interviewing 86 victims and witnesses of violence, the US-based human rights watchdog said it had dozens of unconfirmed reports of killings and attacks by security forces.
Kabila's victory was upheld even after international observers decried electoral conditions, citing problems in the vote count and the loss of huge numbers of ballots.
HRW said 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 Kabila's presidential guard for blame.
"The UN and Congo's international partners should urgently demand that the government rein in its security forces."
London-based rights group Amnesty International earlier this week denounced what it said was a wave of political arrests, notably of opposition activists, since the elections.
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