People in Nigeria headed to the polls Saturday for legislative elections, the first to be held in Africa's most populous country since its transition to civilian rule four years ago.

Many Nigerians complained the voting got off to a slow start, with a large number of polling stations opening several hours late on Saturday.

Officials with Nigeria's Independent National Electoral Commission said they had struggled through the night to get thousands of ballot boxes to 150,000 polling stations.

Heavy rain in the capital, Abuja, and in the main city, Lagos, compounded the delays. Still, observers said the turnout appeared high in the early hours of voting.

More than 50 million people were eligible to vote. Nigerians chose members of the 360-seat House of Representatives and 109 Senators in what is the country's first legislative poll in 20 years.

These elections, and the presidential poll that will follow on April 19, are seen as a test of the strength of Nigeria's four-year-old democracy, following its transition from military to civilian rule in 1999.

President Olusegun Obasanjo was among those turning out to vote Saturday. The Nigerian leader, who will seek re-election, cast his ballot in his hometown of Abeoukuta, north of Lagos.

Analysts worried the election might be marred by violence and fraud following a series of ethnic clashes, mainly in the oil-producing areas of the Niger River Delta.

Members of the Ijaw group last month battled with the military in a dispute over electoral boundaries. Some Ijaws accused members of the rival Itsekeri group of trying to monopolize political power in the region.

On Friday, Ijaw militants threatened to block voting in the region to press their demands for the electoral districts to be redrawn.

In the opposition-held north, political leaders called on President Obasanjo to postpone the election.

The Obasanjo government vowed the elections would go as scheduled, and ordered security forces to deploy in areas across the country.