At least 24 people have been killed by police in the Democratic Republic of Congo since last month's disputed elections, a US rights group has said. President Joseph Kabila was sworn in for a second term this week, but his rival, Etienne Tshisekedi says he won. Human Rights Watch (HRW) said most of the deaths occurred in the capital, Kinshasa, where Mr Tshisekedi has strong support. It said it had evidence security forces had tried to cover up the deaths. The elections were the first to be organised locally since the end of a devastating civil war in 2003, which left some four million people dead. The results were announced on 9 December, but were immediately rejected by Mr Tshisekedi and his supporters, leading to clashes in the capital. International observers said the electoral process had been too flawed to be credible. HRW said that the deaths and the "arbitrary" arrest of dozens of people appeared to be an attempt by the authorities to stifle protest against the results. Based on eyewitness accounts by HRW staff and Congolese activists, the group said it had evidence that 20 people had died in Kinshasa, two in North Kivu, and two in Kasai Occidental province. Among the alleged victims were opposition supporters attempting to protest or simply bystanders, including a 13-year-old boy shot as he was standing outside his home and a woman killed while she was looking for her children. "These bloody tactics further undermine the electoral process and leave the impression that the government will do whatever it takes to stay in power," said Anneke Van Woudenberg, senior Africa researcher at HRW. HRW said security forces appeared to be covering up the scale of the killings by removing bodies and telling medical personnel not to give out information about such deaths. In addition, it accused the Republican Guard - the military unit which is meant to protect the president - of acting outside its authority by arresting civilians and holding them at "illegal places of detention". "The government should order the immediate release of all detainees in their custody, and undertake an impartial investigation into responsibility for these unlawful arrests and the mistreatment of detainees," said Ms Van Woudenberg. Mr Kabila dismissed similar accusations made against the security forces at the end of the electoral campaign as unproven. He has congratulated the police and the army on their patriotism and professionalism in maintaining order before, during and after the election.