The United States and Britain have said that Nigeria's election results "may be subject to deliberate political interference", a harsh warning issued just as the official vote count began in a crucial election for Africa's most populous country.

The statement on Monday from US Secretary of State John Kerry and British Foreign Secretary Philip Hammond said there were "disturbing indications" that such interference would mar the bitterly contested race.

The final election results from Saturday's election are expected on Tuesday, but concerns of fraud could delay the process. The country's election commission started releasing its state-by-state tally of vote counts on Monday afternoon, but it later announced that it would investigate allegations of electoral fraud in at least one state.

The vote marks a pivotal moment for this democracy of 170 million and for the region as a whole. Many observers worry that an election perceived as fraudulent could lead to violence and bitter divisions in the country as it takes on a persistent insurgency and long-standing economic problems.

After the 2011 vote, hundreds of people were killed in clashes in the northern state of Kaduna, where staunch supporters of both parties live in close proximity. Those elections were riddled with problems, but the race was not nearly as tight as it is this year.

Nigerian politics is still largely divided along religious and geographic lines. President Goodluck Jonathan, a Christian from the south, and challenger Muhammadu Buhari, a Muslim from the north, were expected to do well in their traditional strongholds. But frustration with the current administration, which in some places cuts across those divides, might give Mr Buhari an upper hand.

With about 15 million votes tabulated, about half of the total, Mr Buhari was ahead of Mr Jonathan by roughly two million votes. But the vote counts from several states key to Mr Jonathan's campaign still had not been announced.

On Monday, an outbreak of violence in Kaduna provided a sign of the kind of trouble many Nigerians fear. Soldiers shot into a crowd of young men on the side of the road here, killing one and wounding four, witnesses said. The body of 20-year-old Omar Abdul Bakar, covered in a blue shroud, was carried to a nearby mosque.

The men said the incident began after they had closed the gate to a private road leading into their community, fearing that outsiders angry about the election results could enter. When the soldiers demanded entrance, the clash erupted.

For now, the shooting in Kaduna appeared to be a relatively isolated case of election-linked violence. But many here voiced concerns about unrest after the election results are announced, no matter who is declared the winner.

"The ordinary people will go to the street. It's a mob mentality", said Shehu Sani, a senator, referring to would might happen if Mr Jonathan is announced the winner in an election considered fraudulent by the opposition.

If Mr Buhari is declared the victor, many worry about possible violence in the oil-producing south. Militants in the region have already warned that they won't accept such a result.

"It will be a great internal nation-building challenge to convince people in the Niger Delta that he is everyone's president and that the age of divisiveness over," said Carl LeVan, a Nigeria expert at American University.

An incumbent has never lost a presidential election in the 16 years since Nigeria transitioned from military rule. As results trickled in, many Nigerians watched on television and scrolled through their social media feeds to get the latest information. In Kaduna, few businesses opened as residents braced for a strong public reaction.

The joint American and British statement said that there had been no signs of "systemic manipulation of the (electoral) process," but that there were "disturbing indications" that the vote counting could be tainted by political interference. It didn't suggest the scale of the possible fraud or name the parties who might be responsible.

Voting was delayed by six weeks as the government launched an offensive against the extremist group Boko Haram. But while that operation – backed by Niger, Cameroon and Chad – appears to have driven the Islamist militants from important strongholds in the north, it will be up to the next president to root out the militants from their rural hideouts.

Over 10,000 people have been killed in violence related to Boko Haram in recent years.