Four people have been killed in the DR Congo capital Kinshasa after incumbent Joseph Kabila's victory in presidential polls was announced on Friday. Police shot dead three looters, with one woman hit by a stray bullet, according to national police chief General Charles Bisengimana. Since the results, there have also been unconfirmed reports of deaths in Mbuji-Mayi and North Kivu. The main opposition candidate Etienne Tshisekedi has also claimed victory. The government branded Mr Tshisekedi's move "illegal and irresponsible". 'Impossible turnout' The Carter Center, which had 26 teams of observers monitoring the elections, said the results "lack[ed] credibility". It criticised irregularities, including the loss of nearly 2,000 polling station results in Kinshasa, where Mr Tshisekedi has strong support. And it said some constituencies in Katanga province "reported impossibly high rates of 99 to 100% voter turnout with all, or nearly all, votes going to incumbent President Joseph Kabila". Mr Tshisekedi, along with the EU, the US, the UK, France and ex-colonial power Belgium, has appealed for calm. Riot police are patrolling the capital, Kinshasa, and gunshots have been heard. The official results gave President Kabila 49% of the vote against 32% for Mr Tshisekedi. The 78-year-old opposition leader said the results were a "provocation". "It is scandalous and vulgar. We have done our own calculations and I received 54% to Kabila's 26%. His term is finished. I am the president," Mr Tshisekedi said. He later appealed to his supporters to "stay calm and peaceful". Nonetheless Information Minister Lambert Mende said the remarks could spark violence. "We must firmly condemn the self-declaration by Mr Etienne Tshisekedi," AFP news agency quotes him as telling journalists. "It's an irresponsible act that violates the laws of the republic." UN Secretary-General Ban Ki-moon called for "any differences regarding the provisional results of the polls to be resolved peacefully through available legal and mediation mechanisms". 'Flawed, not fraudulent' The army says it has about 20,000 soldiers on standby in Kinshasa. The atmosphere in the city is said to be tense. Kinshasa, in the extreme west of the country, is an opposition stronghold and overnight columns of smoke were seen rising over districts backing Mr Tshisekedi as groups of young men burned tyres. "Since last night it has been very bad, we haven't been able to sleep because of the gunfire," Tresor Nkuna, a resident of Kinshasa, told Reuters news agency. Many shops and stalls in the city's markets have been closed for most of the week as the country awaited the results. They had been delayed since Tuesday, with election officials blaming logistical problems. In areas loyal to President Kabila, mainly in the east of the country, residents cheered and supporters staged victory parades. Alphonse Kasuasua, a civil society leader in East Kasai province where support for Mr Tshisekedi is strong, said security forces were using live bullets to deal with his angry supporters who had taken to the streets. "I tried to go out this morning but the security forces were firing to force people back into their homes," he said. International observers said the vote was flawed but stopped short of calling it fraudulent. Most said the irregularities were not enough to change the outcome. Four other candidates have said the election was rigged and should be annulled. Deadly clashes marred the period leading up to the election and thousands of foreigners and Congolese have fled the country for fear of further violence. Mr Kabila, 40, has been president since 2001 following the death of his father, Laurent. In 2006 he won the first elections since the end of a five-year conflict and is due to be sworn in on 20 December for his second term. But his victory must first be confirmed by the supreme court.