Johannesburg - Nigerian ex-militant group leader Henry Okah has been arrested in Johannesburg, with his lawyer saying it was "unwarranted" and calling for his immediate release.

"Although Mr Okah, a Nigerian citizen, is a known sympathiser of the fate of the people of the Niger Delta, he denies having made himself guilty of any crime in South Africa or anywhere else," Piet du Plessis said in a statement on Saturday night.

Okah was arrested on Saturday evening after a dual car bombing killed 12 people in Abuja, Nigeria on Friday.

He had been linked to the Movement for the Emancipation of the Niger Delta (Mend), but the group denied he had any involvement in the attack.

Du Plessis said it should be noted that Nigeria's President Goodluck Jonathan had already stated that Mend and the people of the Delta were not to be blamed for the blasts.

"Our instructions are that his arrest is totally unwarranted and only an ill-advised attempt to please the Nigerian authorities," he said.

The Associated Press earlier reported that South African police spokesperson Tumi Shai had said police had no comment on the matter, referring queries to the Nigerian government.

Foreign affairs department spokesperson Saul Molobi also refused to comment.

Du Plessis hoped South Africa would not fail Okah again as, a few years ago, he was apparently unlawfully arrested and detained in Angola and South Africa, which resulted in him being extradited to Nigeria.

- SAPA