A prominent Nigerian rights lawyer kidnapped by gunmen in a raid that left four policemen dead last month has been released after almost three weeks, a police spokesman said Thursday.

Mike Ozekhome, kidnapped in his home state of Edo in southern Nigeria on August 23, "was released unharmed late yesterday by his abductors," Moses Eguavoen of the Edo state police command told AFP.

The gunmen opened fire and killed four policemen who attempted to prevent the abduction of the lawyer on a road in the area, the police said.

Eguavoen said he did not know the circumstances surrounding his freedom but said that "the police have launched an aggressive manhunt in a bid to arrest his abductors."

He could not confirm if a ransom was paid before the Lagos-based lawyer was released.

Ransom kidnappings occur regularly in southern Nigeria.

Most victims are released unharmed after the payment of a ransom, unlike in the country's north, where Islamist extremists have killed a number of their hostages.