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’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.

There were no official casualty figures, but reports suggested there were deaths.

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 and parts of the capital, Abuja.