President Muhammadu Buhari extended his lead as votes were being counted in Nigeria's General Election, amid allegations of manipulation.

President Buhari had won 13 of Nigeria's 36 states, while his rival Atiku Abubakar had taken 11 states and the capital, Abuja.

As results came in, Mr Abubakar's People's Democratic Party (PDP) alleged that there have been irregularities.

Party chair Uche Secondus called the count "incorrect and unacceptable".

He said there had been an "attempt by the government and other agencies to manipulate the result", but did not give any evidence.

After the results from 24 states and the capital had been declared, President Buhari had a 1.5 million-vote lead, but the results from the states in the south-east, the PDP heartland, had not yet been announced.

Strong challenge

The EU, US and the African Union have all expressed concern about delays and logistical problems with voting on Saturday, but no independent observers have suggested fraud.

President Buhari, a member of the governing All Progressives Congress (APC) party, is seeking re-election but faces a strong challenge from Mr Abubakar.

Each party says the other is working with the Independent National Electoral Commission (INEC) to influence the vote, which was initially scheduled for February 16 but delayed at the last minute.

President Buhari has promised to build on his accomplishments in office so far, while Mr Abubakar, a business tycoon, has accused the president of wasting his term.

Whoever wins in Africa's most populous nation and largest economy faces a range of problems including power shortages, corruption, security threats, and an economic slowdown.

People were killed

Final results are not expected until later in the week. But Mr Secondus has criticised the initial tallies, saying the government was using "impeachments, manipulation, incarceration" to influence the outcome.

In turn, the federal government accused the PDP of trying to "scuttle the polls" and prompt a constitutional crisis.

The initial vote was postponed, five hours before polls were due to open. Voters were also choosing members of the House of Representatives and Senate.

Most of the country was calm but there were reports of attacks by the Boko Haram Islamist militant group in the north, and voter intimidation and attempts to steal ballot boxes, especially in the southern states of Rivers, Lagos, and Anambra.

A coalition of civil society groups reported that a total of 16 people were killed around the country - fewer than in previous elections.