Henry Okah is widely believed to be the leader of the Movement for the Emancipation of the Niger Delta (MEND), whose campaign of sabotage has cut Nigeria's oil output by around a fifth since early 2006, helping drive up world crude prices.

Okah, who still commands loyalty from several well-armed factions in the delta, is on trial in camera in the central city of Jos. He could face the death penalty if convicted.

Wilson Ajuwa, one of Okah's lawyers, said his legal team was seeking testimony in court from Obasanjo, his army chief of staff and representatives of international oil firms including Royal Dutch Shell and Agip.

"We filed the subpoena last week. It has been signed by the court, so we are serving it now," Ajuwa told Reuters.

Obasanjo, a former military ruler who regularly disregarded court orders during his time in office, was serving his second term as civilian president when MEND began bombing oil pipelines and kidnapping foreign oil workers more than two years ago.

TRIAL IN SECRET

The group has made Okah's release a condition for suspending its campaign of sabotage and for taking part in peace talks planned by the government of President Umaru Yar'Adua, who took power as Obasanjo's elected successor in May last year.

Okah's trial was adjourned for a second time on Monday to later this month after the prosecution amended its list of charges to 62 from 55 and the defence argued that meant their client should have another chance to enter his plea.

Okah's defence team are also pursuing a bid to have the case heard in public. His lawyers have said that appeal will be heard by a separate court in September.

The secrecy of the trial has angered Okah's supporters, who say senior politicians fear a public hearing might expose their own involvement in the unrest in the Niger Delta, the hub of Nigeria's 2.1 million barrels per day oil industry.

Rights groups say gangs behind kidnappings, oil theft and other violent crime in the region have gone unpunished partly because of their connections to local politicians who used them to intimidate opponents or rig elections.

The authorities say Okah's trial must be held in secret for the sake of national security in the world's eighth biggest oil exporter.