President Muhammadu Buhari, who is running for a second term, has arrived at his polling station in Daura, in the far north of the country, one of his assistants has tweeted.

Mr Buhari is facing 72 other candidates, but his main rival is Atiku Abubakar of the People's Democratic Party (PDP).

The election process has now started in Taraba state, in the east of the country.

Our reporter there, Sani Sulaiman, snapped the scene shortly before things got going as officials were sorting out the election material.

Chris Ewokor
BBC Africa, Abuja

After getting a glimpse of the voters queuing outside the polling station at LEA primary school in the Abuja suburb, Dutse Alhaji, let's take a look at what was happening inside just before polls opened:.

The workers were getting up after spending the night sleeping inside the polling station.

They were promised foam mattresses but were just given thin mats.

Regardless, they appeared in high spirits.

Nigeria is Africa's most populous nation with 196 million people and the numbers involved in this vote are big:

• 84 million people have registered to vote
• 73 million collected their voter's card
• More than 50% of the electorate are under the age of 35
• 4% are over the age of 70
• More than 25% of registered voters are students
• There are nearly 120,000 polling stations
• 73 candidates have registered for the presidential election
• Voters will cast three ballots: one for president, one for senator and one for the member of the House of Representatives

Chris Ewokor
BBC Africa, Abuja

Many of the voters here in Dutse Alhaji, a suburb of the capital, Abuja, say they arrived the polling station before 06:00 (05:00 GMT) to ensure they cast their ballots first.

The turnout has been described by people here as unprecedented.

About two hours before polls opened, residents of the town of Maiduguri, the capital of Borno state in the country's north-east, reported hearing several blasts and gunfire.

State police said in a statement that there was no threat to public safety.

"The gunfire were not targeted at members of the public but was for security purposes," the statement said.

In the north-eastern state of Yobe, suspected Islamist militants attacked the town of Geidam, forcing people to flee, the Reuters news agency reported.

Image source, Getty Images

Here's a quick guide on how to vote:

• Join the queue at a polling unit. Present yourself to the electoral official there. They will check your Permanent Voter Card (PVC) photo. Then you'll be directed to another official.

Image source, Getty Images

• The official will check your PVC using a smart card reader. Place your finger on the machine to verify your identity. The reader can access details of all those registered at the polling unit.

Image source, Getty Images

• Give the next official your card and they will check your name and details are in the Voters Register. Your name will be ticked off the list.

Image source, Getty Images

• The official will apply indelible ink to the front of your finger to show you have been accredited to vote. You will not be allowed to vote if your name is not on the register.

Image source, Getty Images

• The polling officer will give you a stamped, dated and signed ballot paper and show you to a voting booth, where you can vote in secret.

Image source, Getty Images

• Stain your fingertip with the ink provided and mark the space in the box on the ballot paper next to who you want to vote for.

Image source, Getty Images

• Roll the ballot paper, leave the booth and put the paper in the ballot box.

Image source, Getty Images

• Leave the polling unit - or stay to watch the process, vote-counting and declaration of the results, which will be displayed at each unit.

Image source, Getty Images

Polls in Nigeria's election are due to open at 08:00 (07:00 GMT) and close at 14:00 (13:00).

For those more used to dawn to dusk voting times these sound very short, but the electoral commission says that anyone still in the queue when the polls shut can still vote.

The first results are expected to come out by Sunday and then the final result could be announced late on Monday, or early Tuesday morning, BBC Abuja editor Aliyu Tanko says.

The candidate with the most votes is declared the winner in the first round, as long as he gains at least 25% of the votes in two-thirds of Nigeria's 36 states.

Image source, Reuters

Nigerians are finally going to the polls to vote for their president following a last-minute postponement of the election a week ago.

The main challenger to President Muhammadu Buhari, 76, is former Vice-President Atiku Abubakar, 72.

Mr Buhari says he has built a strong foundation for prosperity but his rival says Nigeria is not functioning.

Whoever wins will have to address power shortages, corruption, security problems and a sluggish economy.

Read the BBC News story for more.