By Tume Ahemba

LAGOS, Aug 30 (Reuters) - A Nigerian opposition candidate challenging the election of Senate President David Mark has been threatened with death unless he withdraws his petition, his party said on Thursday.

April's general elections in Africa's most populous nation were so heavily marred by vote-rigging and ballot-stuffing that European Union observers said they fell "far below basic international standards" and were "not credible."

Mark's victory was controversial because local media reported that initial results in his constituency showed Usman Abubakar, of the opposition All Nigeria People's Party (ANPP), had won a crushing victory, only for the central electoral body to declare Mark winner.

Abubakar, better known as "Young Alhaji", has contested the results in an electoral tribunal in the north-central state of Benue, and has come under enormous pressure to withdraw.

"The letter (to Abubakar) was written in red ink, we don't know who the author is, but they threatened to kill him if he does not withdraw the case at the election tribunal," ANPP state chairman Godwin Ukpoju told Reuters by telephone.

Other senior members of the party in the state have also received similar threats by text message, and the threats have been reported to the police, he added.

"We remain undeterred until the tribunal passes its verdict," Ukpoju said.

A local police spokesman was not available for comment.

A spokesman for Mark declined to comment.

Political assassination is relatively common in Nigeria, and at least three politicians planning to run for state governorship posts were murdered before the April polls. Many more prospective candidates for local government positions were also killed.

HUNDREDS OF CASES

Abubakar's case is one of hundreds of legal challenges by opposition parties against the landslide victory of President Umaru Yar'Adua and his People's Democratic Party (PDP) at election tribunals across the country.

The April elections were meant to be a milestone for Africa's top oil producer, marking the first handover from one elected leader to another in Nigeria's 47-year post-independence history.

But results switching, violence and intimidation -- mostly by the PDP -- were so widespread that the main domestic observer group called the vote a "charade".

President Umaru Yar'Adua has said he believes he has the mandate of the Nigerian people and promised to let the tribunals probe abuses without interference.

Mark's supporters have persuaded tribal chiefs in Benue state to back him and there have been a series of clashes between rival groups at the venue of the tribunal in the state capital Makurdi.

Mark, a former army general, told EU observers who met him last week that the elections had been "misunderstood" by some in the international community.

Yar'Adua, who acknowledged during his inauguration on May 29 that the elections had shortcomings, has set up a 21-member panel to reform the electoral process.

But critics say the move is premature until legal challenges are resolved.

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