Nigeria's two main presidential candidates have signed an agreement to prevent violence in tightly contested elections due on Saturday.

Ex-military ruler Abdulsalami Abubakar brokered the deal in talks between President Goodluck Jonathan and his main challenger Muhammadu Buhari.

The two promised to respect the outcome of a credible poll and urged their supporters to refrain from violence.

Some 800 people were killed after the 2011 contest between the two rivals.

Mr Jonathan is facing a strong challenge from Gen Buhari, with some analysts predicting a photo-finish.

Thursday is the final day of campaigning and the government has closed its land and sea borders to ensure a peaceful election.

'Embrace peace'

The meeting between the two candidates came after Nigeria's National Peace Committee, chaired by Gen Abubakar, warned on Monday that campaigning had been marred by hate speech that could trigger a crisis in the oil-rich state.

The BBC's Tomi Oladipo reports from the capital, Abuja, that both candidates shook hands and hugged.

They called on their supporters to embrace peace, regardless of who won.

But concern remains in Nigeria over whether the electoral commission is ready for the huge logistical exercise in sub-Saharan Africa's most populous state, our correspondent says.

The polls were due on 14 February, but were postponed to 28 March to give the commission more time to prepare for the polls and for regional forces to regain territory from militant Islamist group Boko Haram in the north-east.

Campaign group Human Rights Watch says Boko Haram had killed some 1,000 people this year alone.

On Wednesday, army chief Kenneth Minimah said adequate security arrangements had been made for the polls.

Anyone who caused conflict would meet "organised violence" from the security forces, he added.

Meanwhile, Doha-based al-Jazeera reports that two of its journalists, Ahmed Idris and Ali Mustafa, have been detained by government forces in the city of Maiduguri, the former headquarters of Boko Haram.

It quoted the military as saying the journalists, both Nigerian nationals, were operating without "protection, accreditation or due clearance".

Al-Jazeera said both men had been accredited by the electoral commission to report from anywhere in Nigeria and it demanded their unconditional release.