* Opposition says mass arrests meant to prevent free vote
* Authorities say simply enforcing the law
* Akwa Ibom has history of political thuggery
LAGOS, April 7 (Reuters) - Nigerian authorities have arrested scores of opposition members in the southeastern state of Akwa Ibom in a campaign of intimidation ahead of elections, opposition lawyers and rights activists said on Thursday.
More than 40 supporters of the Action Congress of Nigeria (ACN), including governorship candidate James Akpanudoedehe, have been arrested following rioting last month between rival party activists, Akpanudoedehe's lawyer Frank Ekanem said.
The detentions risk triggering further unrest in Akwa Ibom, which lies on the edge of the restive oil-producing Niger Delta, and could taint Nigeria's efforts to hold its first credible and violence-free polls since the end of military rule 12 years ago.
"It is a well-planned ploy to take the Action Congress (ACN) out of the election. Right now they have listed more than 100 people that are wanted," Ekanem told Reuters.
"All of the candidates of the ACN have gone underground. They cannot campaign, they cannot solicit for support, they are running helter-skelter. Many of them have left town," he said.
Africa's most populous nation holds a cycle of elections starting with parliamentary polls on Saturday, presidential elections a week later and governorship votes in its 36 states on April 26. The polls had been due to start last Saturday, but administrative chaos forced a week's delay.
Akwa Ibom is one of the main potential flashpoints.
The authorities in the state blamed Akpanudoedehe, a former government minister, for instigating violence last month in which at least four people were killed and more than a hundred vehicles set ablaze and buildings vandalised.
"We are not politicians, we are not partisan. We are a disciplined force carrying out our legitimate duties professionally," said police spokesman Onyeka Orji.
"We do not arrest to witch-hunt anyone. We arrested at least 40 persons in connection with the incident and they are now standing trial. We are doing our job and we are not under the control of any politician or political party."
HISTORY OF INTIMIDATION
ACN supporters say the trouble broke out after PDP governor Godswill Akpabio prevented them holding a campaign rally. They say the violence was deliberately orchestrated to frame them.
Akpanudoedehe was initially charged with treason for being a threat to national security, a charge which can lead to life imprisonment and which legal experts said was disproportionate.
He was granted bail by a federal high court in the capital Abuja but immediately re-arrested and flown back to Akwa Ibom, where he was charged in a local Magistrate's Court with eight counts including murder, arson and conspiracy, Ekanem said.
He is still being held in police custody despite another federal high court ruling that he should be released.
Human rights campaigners have long accused incumbent governor Akpabio's administration of using the threat of violence to intimidate opponents.
"The signs have been there that the incumbent governor did not want any kind of opposition. For most of 2010, anybody that showed interest in contesting an election ... was attacked or their relative or wife kidnapped," said Anyakwee Nsirimovu of Niger Delta-based Institute for Human Rights & Humanitarian Law.
"There were outright killings and nothing was done to stop that from happening ... What we are seeing now is a manifestation of something that has been there right from the start of the tenure of the current governor."
Akpabio's administration denies any suggestion that it intimidates opponents, describing the accusations as a smear campaign ahead of the elections.
"My governor does not control the police and we do not have a state police, neither do we use any machinery or structure of the state to intimidate anyone, not even the opposition," said Akpabio's spokesman Usoro Usoro.
"Their claims are baseless and a mere campaign of calumny against the state governor." (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Additional reporting by Austin Ekeinde in Port Harcourt; Writing by Nick Tattersall)