Alhaji Mujahid Dokubo Asari, leader of the banned Niger Delta People's Volunteer Force (NDPVF), appeared in court in Abuja, where a judge ordered him to be remanded in custody for up to two weeks.
"The charges that are going to be brought against him are treason and unlawful assembly. We are going to file formal charges against him in the next two weeks at the Federal High Court," Justice Minister Bayo Ojo said outside the court on Thursday.
Asari was furious at not being charged and released. As he was led away he denounced President Olusegun Obasanjo's government as the "height of dictatorship in the land".
The 40-year-old Asari's lawyer appeared alongside him but said he was unable to represent his client has he too faces charges. "I came from Port Harcourt to bail him out and I've been arrested for treason," he shouted to reporters as he was led off.
News of the court appearance will raise tensions in the delta and the southern oil city of Port Harcourt, where Asari's loyalists were meeting to plan their retaliation to Tuesday's arrest.
"We will blow every oil installation up," NDPVF deputy commander Alali Horsefall told AFP by telephone overnight, warning that activists were moving through the creeks of the delta to seize oil installations and a gas plant.
NDPVF activist Erik Osima said: "We're getting together now, you'll know in a few hours what will happen."
The 12 million-strong Ijaw people
are fighting for independence
The group, which campaigns for the independence of the 12 million-strong Ijaw people, on Wednesday declared the start of "240 hours of rage" in protest at Asari's detention.
Oil companies have taken extra security precautions, limiting the movements of staff and withdrawing workers from exposed field operations in the delta, a Scotland-sized swathe of swamp and mangrove forest, company managers said.
The Nigerian military said they were ready for any attack.
"We have enough men to take care of all the key points. If they want to attack that will be to their own disadvantage. We will keep security and keep the peace," said army spokesman Colonel Mohammed Yusuf.
Loud and clear
Justice Minister Ojo was emphatic. "I want to say loudly and clearly: Nobody, no citizen of this country, can hold this country to ransom. Nobody can intimidate the government and the government will never fold its arms and allow people to overawe it," he said.
The NDPVF signed a ceasefire with the federal government in August last year after several months of bloody clashes.
"Nobody, no citizen of this country, can hold this country to ransom. Nobody can intimidate the government and the government will never fold its arms and allow people to overawe it"
Nigerian Justice Minister
Since then the guerrilla force has given up several of its secret bases in the creeks and surrendered thousands of assault rifles and machine guns.
Asari's arrest followed the publication of a newspaper interview in which he said: "Nigeria is an evil entity. It has nothing to stand on and I will continue to fight and try to see that Nigeria dissolves and disintegrates."
Asari regards Nigeria as an artificial state imposed on his people by the former British empire and he has made himself a popular hero among the Ijaw.