(Adds White House statement, colour)

By Barry Moody

ABUJA, April 16 (Reuters) - Nigeria's Supreme Court on Monday raised doubts over the results of flawed state elections last weekend and cleared the way for a last-minute presidential bid by Vice President Atiku Abubakar.

As protests spread over widespread electoral abuses and local and European observers said the balloting was seriously flawed, the court ruled the official electoral body did not have the power to block Abubakar from the Saturday presidential poll. The ruling affected not only Abubakar, who said he would now stand for the opposition in the election, but at least six governorship candidates previously disqualified from the state elections last week. They could now mount legal challenges.

"Governorship elections where any candidates were disqualified by INEC are null and void," said Lai Mohammed, spokesman for Abubakar's Action Congress party.

European Union (EU) observers said they had serious concerns over the April 14 vote and called on the Independent National Electoral Commission (INEC) to ensure major improvements before the presidential poll. Washington seconded the call.

"We would urge that officials address any problems in order to ensure that Saturday's presidential election, that those polls are free and fair and conducted in an atmosphere free of violence," said White House spokeswoman Dana Perino.

"WORSE THAN ROBBERY"

A coalition of civil society observers said malpractice in last Saturday's balloting meant the results in 10 of Nigeria's 36 states, "cannot be said to have reflected the will of the people ... and we therefore reject them".

Abubakar told journalists: "What happened on Saturday was absolutely not an election, it was worse than robbery."

There was no immediate comment from INEC.President Olusegun Obasanjo has tried every possible manoeuvre to block Abubakar, with whom he has engaged in a bitter personal feud for years.

Obasanjo failed to change Nigeria's constitution so he could run for a third term but analysts say he wants to dominate any future government of his ruling People's Democratic Party (PDP).

Opposition parties were due to meet on Tuesday to discuss joining forces under a single candidate against PDP's presidential candidate, Umaru Yar'Adua.Until the court ruling, the strongest opposition challenge was expected from former army strongman Muhammadu Buhari of the All Nigeria People's Party.

In Obasanjo's home town, locals said armed gangs loyal to PDP overran voting stations and stuffed ballot boxes last Saturday. Many talk of boycotting this week's polls -- billed as Nigeria's first fully democratic handover of power since independence from Britain in 1960.

"People are scared. If you speak out they come to your house in the night and attack you," said Coker Aderu Oye, clerk at the whitewashed St. Peter's Cathedral Church, oldest in Nigeria.

LANDSLIDE

Official results on Monday from the state elections showed the PDP headed for a landslide victory.

INEC said the PDP had won 26 of 32 states for which results were announced, after elections marred by violence that killed around 50 people. This was around the same number of states as the PDP had won at this stage in the last elections in 2003.

INEC said it had cancelled results from two states, Imo and Enugu, and they would be rerun within weeks.

The results were greeted by violence, with youths burning electoral and local government offices in the northern Kaduna and Katsina states, the state-run news agency said.

Opposition supporters blocked roads to protest against a PDP victory in the central Kogi and southern Edo and Ondo states.

In Ondo, women stripped to the waist in the streets, in a traditional protest to shame authorities, local television said.

Youths blocked roads with burning tyres in Edo's capital Benin City and toured in minibuses, waving brooms, the symbol of Abubakar's party, Reuters correspondent Estelle Shirbon said.

Police fired shots in the air from their vehicles as they escorted convoys of people who residents said were from the PDP.

Africa's most populous nation and top oil producer returned to democracy in 1999 after decades of army rule. (Additional reporting by Tom Ashby, Tume Ahemba in Lagos, Camillus Eboh and Felix Onuah in Abuja, Estelle Shirbon in Benin City)

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