Nigerian President Goodluck Jonathan has appealed for an end to "unnecessary and avoidable" post-election violence across the north of the country. Incumbent Mr Jonathan has been declared winner in the presidential poll, with the electoral commission saying he received about 57% of the vote. Rioting spread across the Muslim north - the opposition's powerbase - as the outcome became clear. International observers have said the election was reasonably free and fair. But supporters of Mr Jonathan's main rival, Muhammadu Buhari, allege ballot-rigging. The Red Cross says it believes many people have been killed in clashes with the police in northern areas of the country. Homes of supporters of Mr Jonathan, a Christian from the oil-producing Niger Delta and the candidate of the governing People's Democratic Party (PDP), were attacked in the cities of Kano and Kaduna. The BBC's Caroline Duffield, in the Nigerian capital Abuja, says General Buhari himself has yet to comment - but political pressure is growing on him to call for calm as well. In a statement, Mr Jonathan said: "I have received with great sadness the news of sporadic unrest in some parts of the country which are not unconnected with last Saturday's elections. "I appeal to those involved to stop this unnecessary and avoidable conduct, more so at this point in time when a lot of sacrifice has been made by all the citizens of this great country in ensuring the conduct of free and fair elections. "I call on all our political leaders, especially the contestants, to appeal to their supporters to stop further violence in the interest of stability, peace and well-being of this great country. "No-one's political ambition is worth the blood of any Nigerian." Nigeria's Independent National Electoral Commission released final results later on Monday, saying Mr Jonathan had won 22.5 million votes to General Buhari's 12.2 million votes. Mr Jonathan was appointed to the presidency last year upon the death of incumbent Umaru Yar'Adua, whom he had served as vice-president. He staked his reputation on the election, repeatedly promising it would be free and fair. VP's home 'set ablaze' In Kano, the largest city in in the north, homes displaying posters of Mr Jonathan were set on fire, and gangs of young men roamed the streets shouting "Only Buhari!" In Kaduna, where a 24-hour curfew has been declared, youths clashed with the police and military in areas to the north and south of the city, with the security forces firing tear gas and live ammunition. Local TV stations reported that the Kaduna home of Mr Jonathan's running mate, Vice-President Namadi Sambo, was set on fire. They said the city's central prison was attacked and inmates released. A lawyer travelling through Kaduna told the BBC's Focus on Africa he had escaped from a mob in the city. He said youths armed with clubs and machetes were targeting people who did not look like they were indigenous to the north. "My car was damaged [and] the windscreen was broken," he said. "I told my driver... to start the car and take off and at that point they smashed the car. We managed to get away." In the central city of Jos, there is rioting in the Gangare area to the north of the city. There are also reports of violent protests in the states of Gombe, Adamawa, Katsina and Sokoto. And there are fears for the safety of the revered religious leader, the Sultan of Sokoto, who is now facing angry criticism over his support for President Jonathan. While past Nigerian polls have been marred by widespread violence and vote-fixing, Saturday's seemed to go generally smoothly. Voters in many areas queued patiently for hours despite intense heat to cast their votes. Source: BBC