Nigeria's ruling People's Democratic Party (PDP) has taken a commanding lead in early results for weekend state governorship and legislative elections amid widespread reports of violence and irregularities.

Results declared by the Independent National Electoral Commission (INEC) for 21 of Nigeria's 36 states on Monday showed the PDP as victorious in 17 states, with two opposition parties winning in four states.

Saturday’s vote for state governors and legislators will be followed by elections on 21 April for federal legislators and a new president to succeed Olusegun Obasanjo. Assuming the election goes ahead, it will mark the first transfer of power from one elected government to another since Nigeria's independence in 1960. Obasanjo has served two four-year terms.

Analysts saw Saturday’s vote as a reflection of how the presidential poll will likely unfold in Nigeria, Africa’s top oil producer and most populous country with 140 million people, most of whom live in poverty.

Police Inspector General Sunday Ehindero said 21 people were killed across the country in election-related violence at the weekend. They included seven policemen and a civilian shot dead by armed militants opposed to the vote in the main oil industry centre in the southern delta in attacks on two police stations on Friday.

However, reports in the local media indicate that more than 40 people died in violence linked to the vote across the country. Human rights groups say dozens more people had been killed in election-related violence in the months leading up to the polls.

In southeastern Anambra state at least three people were killed in the weekend violence when angry youths protesting the non-arrival of voting materials attacked and burnt the local electoral office in the city of Onitsha. In several voting centres, armed thugs reportedly stole ballot materials, with opposition parties accusing the PDP of being behind the violence.

"What we have seen is a total sham," said Boniface Akunne, a disappointed voter in the town of Ekwulobia, who accused the electoral commission of working for the PDP. "Where opposition is strong, ballot materials were not delivered. Voting only took place where the PDP had strong support."

Anambra Governor Peter Obi, of the opposition All Progressive Grand Alliance, told reporters that no elections took place in his state.

"Investigations and reports available to me amount to the same gloomy story of non-availability of INEC officials," said Obi, whose deputy, Virgy Etiaba, ran against the PDP candidate and declared winner, Andy Uba. "In the few places INEC officials showed up, they had no voting materials and where they had there were no result sheets.”

INEC, while acknowledging some delays in the delivery of voting materials, insists that the elections were successful.

"Information at our disposal shows that the elections were largely free and fair," said INEC spokesman Philip Umeadi Jr.

However, many election observers saw things differently.

“In some places party officials were openly stuffing ballot boxes,” said Chris Albin-Lackey, Nigeria researcher for New York-based Human Rights Watch. He observed voting in southern oil-rich Rivers State.

He said voting there was “perhaps an especially egregious example” of similar problems with voting that took place elsewhere in the country. “There were places where no voting took place and when the results came back [INEC] indicated that 90 percent of all eligible voters had come out and voted for the PDP.”

In two southeastern states, Enugu and Imo, INEC cancelled the votes, alleging that irregularities were of a scale to affect the outcome of the poll. Those elections are to be held again at a later date.

"Obviously the reasons that the gubernatorial elections were cancelled were because of irregularities which substantially affected the results," Umeadi Jr. told reporters.

Many analysts and average Nigerians doubt INEC’s independence. Confidence has eroded since the organisation appeared to acquiesce to government pressure to keep several opposition candidates off the ballot in a number of key statewide races and in the presidential race, Albin-Lackey said.

“It hasn’t demonstrated any independence at all and has generally behaved as defending the interests of the PDP,” he said.