Ken Niweigha was killed 24 hours after being arrested
A Nigerian oil militant leader has been shot dead in police custody.
The Movement for the Emancipation of the Niger Delta (Mend) claim Ken Niweigha known as "Daddy Ken" was executed by the police.
But the police say he was killed in a shootout after he took them to his hideout to surrender his weapons.
Human rights groups say extra-judicial execution is "shockingly common" in Nigeria, where the police lack the capacity to do their jobs effectively.
After his arrest on Tuesday, Mr Niweigha, who led the Egbesu Boys armed group in Bayelsa State, was paraded in front of journalists.
He then agreed to take the police to his hideout in Odi town and surrender all his arms, local media reported police commissioner Onouha Udeka as saying.
"He promised he was going to take us to his hideout in Odi where he hid his weapons," said Mr Udeka.
"But we did not know that he had arranged with his gang to attack us and possibly get him freed. Ken was shot trying to escape."
Mr Niweigha was the only person killed in the shootout, he said.
A spokesman from Mend - an affiliation of armed groups in the oil-rich Niger Delta - accused the police of "summarily executing" Mr Niweigha.
"The Nigerian government in authorising extra-judicial killings is sending a clear message to youths from the region that it is better to die fighting for freedom than be killed by a lawless system of government," Jomo Gbomo said in an e-mail to journalists.
Mr Niweigha had been arrested after 500 women from Odi demanded something should be done to stop militants fleeing from an army operation in neighbouring Delta State coming to the town.
The women told police "Daddy Ken" that was harbouring them.
It is unusual for people to turn in militants as they are often feared or pay Delta communities to keep quiet, but Odi residents said they feared a repeat of an army operation 10 years ago which devastated the town.
US-based rights body Human Rights Watch reported 50 people were killed in that attack, which was provoked by the murder of 12 policemen, allegedly by militants led by Daddy Ken.
Other local accounts put the number of dead at much higher.
A security operation is currently under way in the swamps of Delta State where the military Joint Task Force (JTF) is hunting Mend fighters.
Militant groups in the Niger Delta have flourished amid a lack of governance and rule of law.
They claim to be fighting to help local people benefit from the region's oil wealth but fund their activities with oil theft, extortion and kidnapping.