A militia group from Nigeria's oil-producing Niger Delta region has seized an oil pumping station, in protest at the arrest of its leader.
Asari accused the government of being a dictatorship
More than 100 armed men in boats stormed the Idama flow-station, sources close to the Chevron oil company said.
The occupation came after militia leader Mujahid Dokubu-Asari was remanded in custody for two weeks by a judge in the capital, Abuja.
He wants more control of oil resources for the Ijaw people of the Niger Delta.
The deputy leader of the Niger Delta People's Volunteer Force, Alali Horsefall, said militiamen were moving towards seven other oil installations in the area.
"We will blow up everything. We will set fire to them," he said.
A company source told Reuters news agency that only about 8,000 barrels per day were affected at Idama.
Chevron spokesman Femi Odumabolaims said Chevron opted to shut down production after "youths disarmed the government security forces around it."
The Niger Delta remains one of Nigeria's poorest and least developed regions, although it accounts for most of the 2.4 million barrels of oil produced by Nigeria, Africa's largest oil-producer, each year.
At his first appearance in court since being arrested on Tuesday, Mr Asari said the government was acting like a "high dictatorship" after his lawyer, Uche Okoko, was arrested.
Justice Minister Bayo Ojo said Mr Asari would be charged with "treason and unlawful assembly".
Mr Okoko, the lawyer, is expected to face similar charges - but he was outraged at his treatment.
"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.
A police spokesman said Mr Asari was being held over a newspaper interview, in which he allegedly said he would continue his fight until Nigeria dissolves.
A statement issued by the Niger Delta People's Volunteer Force on Wednesday warned of "grave mayhem" if Mr Asari were not released by early afternoon.
Hundreds of soldiers and riot police are manning checkpoints in Nigeria's oil capital, Port Harcourt, reports Reuters news agency.
Last year, the Niger Delta People's Volunteer Force contributed to a sharp rise in world oil prices when it threatened war against oil companies.