The build-up to the elections in Africa's largest oil-producing country has seen isolated bouts of violence that have left about 70 people dead, human rights groups say.
 
Polls strongly contested
 
Nigerian elections

Under Nigeria's federal constitution, the president and 36 state governors are elected directly to a four-year term, for a maximum of two terms.

There are 24 parties offering candidates for president and almost 50 parties in total. The three largest parties are: People's Democratic Party, All Nigeria People's Party and Action Congress.

Timetable:
April 14
- Election of governors for 36 states and members of state houses of assembly.

April 21 - Election of president and members of the National Assembly, comprising 109 senators and 360 members of the House of Representatives.

May 29 - Olusegun Obasanjo, current president who has already served two terms, hands over to his successor.

Voters on Saturday will select their state leaders, while elections for Nigeria's national legislators and for the presidency are due to take place on April 21.
 
Under Nigeria's federal system, state governors wield great power and control large budgets, meaning leadership of the 36 states is strongly contested.
 
Politicians have traded accusations of corruption and shady dealings, most notably between Obasanjo's ruling party and Atiku Abubakar, the lead opposition figure and vice president, who has fallen out with Obasanjo and been banned from the race by the electoral commission.
 
Abubakar is contesting the ruling in court and the race has grown increasingly bitter.
 
"It is a matter of great regret that some supposedly highly placed individuals have been reported to be fanning the embers of discord, hatred, violence, and destruction," said Obasanjo, without elaborating or naming individuals, in a nationwide address on Friday.
 
Nigeria's voters will head to 120,000 election centres nationwide, with dozens of international observers and hundreds of Nigerian monitors watching the vote.
 
Results are expected within days.