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ABUJA, April 17 (Reuters) - Nigerian Vice President Atiku Abubakar will be able to stand as an opposition candidate in Saturday's presidential election, the official electoral commission said on Tuesday.

The Supreme Court ruled on Monday that the commission had acted illegally by disqualifying Abubakar, arch-rival of outgoing President Olusegun Obasanjo. But it had not been clear until Tuesday's announcement that his name would be able to appear on the ballot.

"The Commission assures every Nigerian that the vice president will be on the ballot," said Philip Umeadi, spokesman for the Independent National Electoral Commission (INEC).

Abubakar is the presidential candidate of the Action Congress party, which won just one of 36 state governorships in regional elections last Saturday that were marred by widespread rigging and violence.

INEC disqualified Abubakar, who has engaged in a bitter public power struggle with Obasanjo for more than two years, because of an indictment for fraud which has been challenged in court.

Abubakar and former military ruler Muhammadu Buhari are the main opposition challengers to Umaru Yar'Adua, Obasanjo's chosen candidate for the ruling People's Democratic Party.

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