European observers judged this year's elections for president, governors and legislators "not credible" because of what they said was widespread vote-rigging and intimidation, mostly by agents of the ruling People's Democratic Party (PDP).

Hundreds of results, from Umaru Yar'Adua's election as president, down to the state house of assembly level, are being contested before election tribunals.

On Thursday, a tribunal in northeastern Adamawa state annulled the election of Murtala Nyako of the PDP as governor, the official News Agency of Nigeria reported.

The opposition Action Congress (AC) said its candidate, Ibrahim Bapatel, had been illegally disqualified from the race after his name appeared on a list of corruption suspects issued by the anti-graft police and endorsed by the government before the elections.

That list was widely denounced at the time as interference in the election process by the PDP government. Since the elections, anti-corruption police have shown no sign of investigating or prosecuting those named on the list.

"The Action Congress has again hailed the judiciary for its continued efforts to ensure the sanctity of elections in Nigeria as well as the survival of the country's democracy," the party said in reaction to the Adamawa ruling.

"We are hopeful that more of the illegalities perpetrated during the last general elections, which have been described as the worst in the nation's history, will come to the fore as other election petition tribunals decide the cases before them."

The AC's presidential candidate, former Vice President Atiku Abubakar, is one of two leading opposition figures contesting Yar'Adua's victory at a tribunal in the capital Abuja, and the AC said Thursday's ruling had encouraged it to continue.

Nigeria returned to civilian rule in 1999 after three decades of almost continuous army dictatorship. Olusegun Obasanjo, a former army head of state from the 1970s, returned to power as a civilian through elections in 1999 and 2003.

The 2007 elections were supposed to be a democratic landmark, the first transition from one civilian president to another through the ballot box since Nigerian independence.

Instead, fraud and violence were so widespread that international and local observers said the results were not credible. Human Rights Watch has called the polls "a farce".