ABUJA, Nigeria — International observers declared Nigeria’s weekend presidential election the fairest in decades. But riots across the north Monday and the loser’s unwillingness to accept the results undercut this divided nation’s bid to move toward orderly democracy.

The violence underscored the West African nation’s deep sectarian divisions. Enraged youths in the mainly Muslim north, stronghold of defeated candidate Muhammadu Buhari, burned down the homes of any northern leader seen as collaborating with Goodluck Jonathan, the victorious incumbent from the mainly Christian south.

Jonathan, of the ruling People’s Democratic Party, won Saturday’s balloting by a wide margin, taking 57 percent of the votes to 31 percent for Buhari, his nearest rival, according to final results.

The results starkly underscored the sectarian divide: The Daily Trust newspaper’s front page ran a map of the vote with the entire south colored blue for Jonathan and the north colored red for Buhari, a former military leader.

Jonathan urged an end to violence and called on other politicians to calm their supporters.

The army was deployed to quell the violence, as smoke hung over the northern city of Kano. In the searing heat, young men drenched in sweat rampaged across the city, carrying iron bars, sticks and swords and lighting fires. Schools and businesses closed, churches were burned, and Christians went into hiding.

The streets were deserted but for the angry mobs chanting pro-Buhari slogans.

Northern leaders of the ruling party were stunned when rioters burned down the home of the sultan of Sokoto, a revered and respected figure for Nigerian Muslims. Another prominent religious leader, the emir of Zaria, fled and his house was burned.

The home of Vice President Namadi Sambo, also a northern Muslim, was burned in Zaria.

The violence began Sunday as results from the Independent Nigerian Electoral Official dribbled out. But the unrest sharply escalated Monday, with reports of rioting in the states of Kano, Kaduna, Niger, Gombe, Bauchi, Sokoto, Yobe, Borno, Adamawa and Plateau state and some neighborhoods in the capital, Abuja. Curfews were imposed in several northern cities.

Five were reported dead in Sunday’s violence.

Human rights activist Shehu Sani, of the Civil Rights Congress, said in a phone interview from Kaduna that his organization had confirmed 42 dead across the north.

“”This is the worst postelection violence I have seen since the return of democracy in 1999,”” Sani said. “”The police shot and killed one person. They became infuriated and went from house to house, killing members of the ruling party and burning their houses.

“”People don’t see it as an election but as a war between north and south. That’s why people are so passionate and angry about it.””

One protester, Hamisu Labaran, 28, held a Buhari campaign poster as he and dozens of others set fire to tires in a Kanu street.

“”Jonathan raised our expectations of a free and fair election but what we have seen is a mockery of democracy,”” Labaran said. “”We are tired of being robbed of our votes in every election for the past 12 years.””

Since military rule ended in 1999, Nigerian elections have been marked by fraud, pre-election violence, threats, underage voting, hijacked ballot boxes and other irregularities.

International observers noted numerous problems in the vote, including some violence, overwhelming crowds which prevented some people from voting, and large regional variations in the number of invalidated ballots. But Joe Clark of the Washington-based National Democratic Institute said the elections marked a significant improvement in Nigeria and a turning point for the country.

“”Any election has to be taken in the context of the country. In the context of Nigeria, these are free, fair and credible elections,”” he said at an Abuja news conference Monday.

Sani and Musa Rafsanjani, head of the Transitional Monitoring Group, a coalition of Nigerian election observation groups, alleged in phone interviews that the riots were sparked by fraud in the election, which saw voter turnout of 54 percent.

Sani said Jonathan must move swiftly to allay northern fears that he would use his tenure to enrich his southern supporters and impoverish the north.

“”People feel that they will not benefit from the president at all,”” Rafsanjani said. “”The youths feel that they have been used by these leaders who go to Abuja and don’t bring any development to the area.””

Analyst Otive Igbuzor, of the African Center for Leadership, Strategy and Development, said in a phone interview some candidates had primed their supporters during their campaigns to protest the results. He said Jonathan must appoint an inclusive government.

“”I think Nigeria has to deal very seriously with the question of managing diversity and how do you run a government in a multi-ethnic and multi-religious society,”” he said.