This post has been updated.

PROTESTS broke out across Nigeria's mostly Muslim north on Monday, as results from the weekend's presidential election seemed almost certain to hand victory to the southern incumbent Goodluck Jonathan.

Of the 39.5m votes cast, Mr Jonathan won 22.5m while General Muhammadu Buhari, a popular northerner and his main challenger, only picked up 12.2m, according to figures from the country's 36 states which have since been confirmed by the national election commission. General Buhari's team has queried some of the results, especially those from some southern states where turnout was over 80%.

As results trickled in on Sunday night, riots broke out in the remote north-eastern states. By Monday afternoon the trouble had spread to Kano and Kaduna, key northern business and political hubs that have lost their shine as the region has declined.

In Kano, youths tried to burn down the home of a traditional Islamic ruler thought to be close to the ruling People's Democratic Party (PDP). In Kaduna, they set fire to the house of Namadi Sambo, the vice-president. Curfews have since been imposed in both of these states and several others.

This month's presidential race has stoked long-existing faultlines in Africa's most populous nation, home to 150m people and over 250 ethnic groups. Mr Jonathan hails from the oil-rich and mostly Christian southern delta. General Buhari is an austere former military ruler from the Hausa ethnic group that dominates the north.

Some northerners said the whole thing had been a dangerous misunderstanding. "These youths have only seen the huge turnout for Buhari in their neighbourhoods and they don't understand that that has not happened elsewhere," said Audu Grema, a development consultant living in Kano. The higher poverty and lower education levels that blight the landlocked north had perhaps caused just as much of the fury, he added.

General Buhari did not call the protesters out, said Yinka Odumakin, the challenger's spokesman. "These people were just reacting to the situation." Both Mr Jonathan and General Buhari have appealed for calm.

On Monday morning in Abuja, Nigeria's manicured capital, international observers had heaped praise on the presidential polls. These elections have been widely hailed as a great improvement on the series of violent and rigged polls that have kept the PDP in power for a dozen years. But the riots that were taking place at the same time, just a few hours' drive away, were a reminder that whoever wins the race will still have much work to do.