By Tume Ahemba

LAGOS, Oct 2 (Reuters) - Police have arrested five members of the ruling People's Democratic Party (PDP) in southwestern Nigeria on suspicion of gang-raping a teenage schoolgirl, a police spokesman said on Tuesday.

The 17-year-old was abducted and raped 10 days ago in Ilesa town in Osun state after she refused to testify against an opposition party at an election tribunal, according to local media and the opposition party.

Hundreds of petitions are being heard across Africa's most populous country to resolve disputes arising from elections last April that were so badly rigged they were deemed not credible by international observers.

"The girl was raped and given inhuman treatment," a spokesman for the police in Osun said by telephone from the state capital Osogbo.

He said the five suspects, all members of the PDP, will soon be charged. Four other suspects are still on the run.

Reports of the rape have sparked outrage, with one local women's group threatening to stage street demonstrations.

Spokesman for PDP in Abuja was not available to comment.

The Action Congress party, which has challenged the election result in Osun that gave sitting Governor Olagunsoye Oyinlola victory, said the rape was part of a plan to muzzle opposition in the state.

"The girl's only offence is that she was present when a PDP chieftain was committing an electoral fraud in Ilesa during the April election," AC spokesman Lai Mohammed said.

"This is not just sheer criminality, it is bestiality to gang-rape a 17-year-old secondary school girl," Mohammed said.

April polls were billed as a watershed for Nigeria, marking the first democratic transfer of power from one civilian to another in its 47-year post-independence history.

But fraud, violence and intimidation -- mostly by the PDP -- were so widespread the main local observer group called the vote a "charade".

President Umaru Yar'Adua, whose landslide victory is being challenged by the opposition, acknowledged shortcomings in the elections but said he believed he had the mandate of a majority of Nigerians. He has pledged to reform the electoral process.

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