At least 50 people have been reported dead in northern Nigeria following widespread post-election violence in the country. Witnesses and rescue workers said the deaths occurred in major cities in the north alone, when riots erupted after incumbent Goodluck Jonathan was declared the winner in the presidential election. Jonathan has condemned the fighting and called for peace. "It is regrettable that when the international observers are condemning us for credible elections, we witness violence in some parts of the country, it is really regrettable because it is uncalled for," he said. His rival, a northerner and a former military ruler, Muhammadu Buhari, has said the vote was rigged but he has also appealed for calm and called for an end to the fighting. Chaos and violence Despite the calls, Al Jazeera's Yvonne Ndege, reporting from the Nigerian capital, Abuja, said reporters are being inundated with eyewitness accounts of chaos and violence, particularly in the northern state of Kaduna. "What we're hearing is that the violence is taking a new dimension," she said on Wednesday. "Eyewitnesses are telling us that soldiers - or men dressed in military fatigues - are conducting door-to-door searches, removing people from their homes and actually attacking and in some cases killing them." She said Al Jazeera had also received reports of attacks on mosques and Muslims in the south of Kaduna, a day after reports surfaced of attacks on churches by Buhari supporters. "All of this is going to only worsen the sense of fear that many people in Kaduna state have," our correspondent said. The violence comes four days after Jonathan, who heads the Peoples Democratic Party (PDP), won Saturday's presidential election. "There are millions of people in Nigeria, particularly in northern Nigeria, who did not want Goodluck Jonathan to run in 2011," our correspondent said. "There is an informal agreement that rotates the presidency between northerners and southerners and it was felt that it was the turn of northerners to hold the presidency in 2011." 'Flawed' vote Following the election results, some Nigerians said the vote had been flawed. Despite those allegations, international observers described the poll as the fairest Nigeria has held in decades. John Kufuor, a former president of Ghana and the lead election observer from the African Union, told Al Jazeera that he was "taken aback" by reports of violence as he had observed the elections take place "credibly, peacefully and transparently". But hours before Jonathan was declared the winner on Monday, Buhari, who stood on the platform of the Congress for Progressive Change (CPC), told Al Jazeera he believed the election had been systemically rigged. "We have evidence in our hands that the computers [used in the voting process], were programmed to produce rigged results," he said. Source:Al Jazeera and agencies