The Nigerian government released a prominent militant leader in the oil- producing Niger Delta late on Monday, hoping to defuse a worsening spate of violence that had led hours earlier to an unprecedented attack on the edge of Lagos.

Lawyers said Henry Okah, who had faced treason and gun-running charges, was freed from jail after becoming the first militant leader to accept an amnesty offer made by Umaru Yar’Adua, the president, last month.