ABUJA, Nigeria — The governing party candidate was declared the landslide winner of Nigeria’s deeply flawed presidential elections yesterday as the runner-up rejected the results, setting the stage for greater strife in Africa’s most populous nation.The elections were meant to boost civilian rule and stability in Africa’s top oil producer, where some 15,000 people have died in political violence since 1999 as factions vied for power in a political space liberated by the end of strict military rule that year.

Questions about their legitimacy undermined the elections arranging Nigeria’s first transfer of power from one elected civilian to another.

President-elect Umaru Yar’Adua, the 56-year old governor of a heavily Muslim northern state, is scheduled to take over the presidency on May 29. But the top opposition leader, Gen. Muhammadu Buhari, European Union and US observers and the United States government all heaped opprobrium on the vote.

Buhari, an ‘80’s-era military dictator, called the outcome “the most blatantly rigged election results ever produced in Nigeria.”

During Saturday’s presidential and parliamentary votes and a week earlier during elections for state governors and legislatures, electoral officials could be seen pressing their thumbs on ballots and shoving them into boxes. Thugs intimidated voters. The presidential ballots bore no serial numbers, making them easy to mishandle and impossible to track.

The European Union monitoring body found the elections “have fallen far short of basic international and regional standards for democratic elections,” said mission head Max Van den Berg.

“I am disappointed,” he told reporters. “These elections have not lived up to the hopes and expectations of the Nigerian people and the process cannot be considered to have been credible.”

In the United States, which counts on Nigeria as a top supplier of oil, State Department spokesman Sean McCormack said: “Based on the reports that we have seen, these were flawed elections and in some cases deeply flawed elections.”

The International Republican Institute, a leading U.S.-based group that sent 59 observers to Nigeria, said the electoral process did not meet international standards. The Transition Monitoring Group, an influential, homegrown group claiming 50,000 Nigerian observers, called for a cancellation of the vote.

In a nationwide address ahead of yesterday’s announcement, President Olusegun Obasanjo, who had been barred from running by term limits after two terms in power, accused the political opposition of “fanning the embers of hate” and engaging in “outright subversive activities.”

Obasanjo acknowledged that the vote had been imperfect, but said Nigerians were nonetheless devoted to democracy. He said any losers should redress any grievances through the courts.

That was exactly the plan, said representatives of Vice President Atiku Abubakar, a one-time Obasanjo ally who fell out with his boss before moving to an opposition party. He ran as a leading opposition candidate and placed a distant third, according to the official results.

Obasanjo, a former military ruler, won a 1999 election that ended 15 years of near-constant military rule. His 2003 re-election was marked by allegations of massive vote rigging.

Dozens of Nigerians have died in civil strife related to these elections, and the outcome seemed unlikely to stanch further bloodshed, like a low-intensity armed struggle in the country’s oil-producing region.

Gunmen battled security forces yesterday in the south, leaving at least seven people dead in the area’s main city, Port Harcourt, police said.