KANO, Nigeria (AFP) – Riots spread across Nigeria's north on Monday as results from presidential elections showed incumbent Goodluck Jonathan assured of victory in polls that reflected deep regional tensions.

Residents reported that a home belonging to an aide to the vice president was among those set alight and a mob sought to burn a Christian woman alive in one area.

Protesters fought running battles with soldiers in Nigeria's main northern city. A 24-hour curfew was imposed on the state of Kaduna, the governor announced on radio.

Rioting began in various states after allegations of vote rigging in Saturday's vote as results showed sharp divisions between the mainly Muslim north and predominately Christian south of Africa's most populous nation.

Observers have hailed the conduct of the poll, describing it as a major change from years of rigged ballots in the continent's largest oil producer.

But concerns have also been raised over extraordinarily high results for Jonathan in parts of his native south, including 99.63 percent in his home state of Bayelsa.

In Kano, Nigeria's second-largest city, protests spread, with mobs armed with daggers, sticks and wooden planks taking up positions as police or soldiers sought to contain them. A shopping complex was set alight.

Protesters stopped cars and demanded that passengers express support for Jonathan's main challenger, ex-military ruler Muhammadu Buhari, whose stronghold is in the north.

A mob used wooden planks to beat two people who appeared to be Christians based on their dress, an AFP correspondent said. Bonfires were burned in the streets while schools and shops closed, with smoke rising above the city.

Protests also spread to the tense central Nigerian city of Jos, hit by years of deadly clashes between Christian and Muslim ethnic groups,

Soldiers fired into the air and two military helicopters buzzed overhead as protesters took to the streets in one of the mainly Muslim areas of the city, an AFP correspondent reported.

In the town of Potiskum in Yobe state, a resident reported that a mob sought to pin down a Christian woman and burn her with a tyre, but residents put the fire out and took her to hospital.

"Youths have been going about setting bonfires on the streets and burning the houses of (ruling party) supporters," resident Kabiru Usman said by phone.

"Soldiers have been drafted to tackle the situation. All streets are deserted save the presence of the mobs."

Official results were still being announced, but Jonathan had built an unbeatable lead that was to see him declared winner in the first round barring any dramatic turn of events.

Millions of voters turned out for Saturday's election as the country of 150 million people bid to put years of rigging and badly flawed ballots behind it and hold the cleanest polls for head of state in nearly two decades.

Analysts spoke well of the organisation of the election on voting day, but questioned figures in certain states showing Jonathan with more than 95 percent.

"Figures of 95 percent and above for one party suggest that these are fabricated figures and, personally, they worry me because they pose serious questions on the credibility of the election," said Jibrin Ibrahim of the Centre for Democracy and Development.

Chidi Odinkalu of the Open Society Justice Initiative told AFP: "Our presidential election was free, fair and truly incredible."

Buhari won by significant margins in parts of the north, but nothing approaching such high figures in selected states for the incumbent.

He had alleged reports of fraud on election day, but his campaign declined to comment on the results so far, saying they preferred to wait until final figures are announced.

Many analysts had feared the deepening of regional divisions that showed up in the results in a country as fractious as Nigeria, roughly split between Christians and Muslims and with some 250 ethnic groups.

Jonathan, 53 and a southern Christian, is the first president from the main oil-producing Niger Delta region.

He had an almost accidental rise to power that culminated with him being thrust into office last year following the death of his predecessor, Umaru Yar'Adua.

Buhari, a 69-year-old Muslim, has built a reputation as a fighter of corruption, but his "war against indiscipline" during his regime in the 1980s was also accused of outrageous rights abuses.

Many in the north saw in him an opportunity to return power to their economically marginalised region.