Henry Okah is believed to be the leader of the Movement for the Emancipation of the Niger Delta (MEND), whose campaign of sabotage against Africa's biggest oil industry has cut output by a fifth, helping to push up global energy prices.
"Bail was refused because of the nature of the charges," Wilson Ajuwa, one of Okah's lawyers, told Reuters. "We are not surprised and we will appeal."
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Okah, who was arrested in Angola in September and extradited to Nigeria to face trial for gun-running and treason, still commands loyalty from several well-armed factions in the delta. He could face the death penalty if convicted.
His trial in camera in the central city of Jos has angered his supporters and prompted MEND to scale up its campaign of attacks on oil facilities in the world's eighth biggest crude exporter.
Okah's lawyers on Friday also filed a new motion for his release on the grounds that his defence team has not had access to witness statements or other prosecution evidence. A hearing has been set for Sept. 19.
MEND, the main militant group in the Niger Delta which produces nearly all of Nigeria's 2 million barrels per day of oil output, has made Okah's release a condition for suspending its campaign of violence launched in early 2006googletag.cmd.push(function() { googletag.display('content2'); });
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