Violent protests broke out in several of Nigeria’s 36 states as partial results from highly contested state elections appeared to hand most of the victories to the governing People’s Democratic Party.

The police said 21 people had died in election violence, but local newspapers estimated that the toll was at least twice that number.

Allegations of vote-rigging, ballot-stealing and intimidation in several states in the first round of Nigeria’s landmark elections set off violence and raised fears that the country’s presidential voting, to be held later this week, could fail, candidates and independent observers said. Of the 10 state governor races for which results were available, the People’s Democratic Party was leading in eight.

“It is a complete disaster,” said Chris Albin-Lackey, a researcher for Human Rights Watch, who spent Election Day on Saturday searching, largely in vain, for voters in the troubled, oil-rich Niger Delta region in the south. “Watching the run-up to these elections, I suppose it shouldn’t be a surprise, but the scale of it is astonishing.”

Government and electoral officials said they were satisfied with the voting, saying it had been largely free and fair. International observers have yet to publicly assess the election.

But as election results and accounts of the voting emerged on Sunday, in many crucial races a picture of widespread incompetence, ballot-stuffing, intimidation and fraud emerged.

The problems are likely to intensify as Nigeria hurtles toward the presidential election, set for April 21, to choose the successor to Olusegun Obasanjo, the former general who has led the country since it emerged from three decades of nearly continuous military rule in 1999. Nigeria is the most populous nation in Africa and its largest oil producer.

The People’s Democratic Party has struggled to retain its dominance as infighting has raged over who would replace Obasanjo.

If the election is successful, it will be the first time in Nigeria that one elected government has handed power to another without military interference, a milestone in a nation plagued by coups, despotism and civil strife.