Thousands of protesters gather at Gani Fawehinmi Park in Lagos
Tens of thousands of Nigerians defied an order to end a three-day-old strike Wednesday as unions threatened to halt output in Africa's top crude producer and a mob rampage left a police officer dead.
The strike over soaring fuel prices sank the continental heavyweight deeper into crisis, with deadly religious violence that saw four Christians gunned down on Wednesday, sparking warnings of a looming civil war.
The tensions have left the global oil market watching anxiously and President Goodluck Jonathan facing his toughest challenge since he was elected last year.
Despite a government order late Tuesday that labelled the strike illegal and threatened to withhold pay, protesters took to the streets as gangs of youths burnt tyres and harassed drivers for cash.
Pockets of Lagos, the largest city in Africa's most populous nation, descended into chaos, including one upscale neighbourhood, with gangs attacking a police car with sticks and ripping down signposts.
The main groups of protesters in Lagos however remained peaceful, with some 10,000 people at one of the largest demonstrations dancing and singing anti-government songs. Some vowed they would begin camping out there.
"I am here with my water and toothbrush because we are not leaving this arena until our demand for fuel at 65 naira ($0.40, 0.30 euros) is met," said Akinola Oyebode, a 23-year-old at the main protest in Lagos.
He was referring to the price of a litre of petrol before government subsidies were controversially scrapped from January 1.
"We shall not be intimidated by the police because our protest is legitimate and constitutional."
In the central city of Minna, a mob went on the rampage, burning political offices, leaving a police officer dead and prompting an all-day curfew. The cause of the violence was not immediately clear.
In Kano, the largest city in the north, a massive crowd estimated in the tens of thousands marched through the streets.
Protesters and police had clashed in Kano on Monday, leaving at least two people shot dead, but no incidents were reported on Wednesday.
Oil production has so far not been affected by the strike, but workers threatened action if the government does not respond to their demands.
"We hereby direct all production platforms to be on red alert in preparation for total production shutdown," PENGASSAN union president Babatunde Ogun said in a statement as union officials met in the oil hub of Port Harcourt.
The other oil workers' union, NUPENG, also threatened a shutdown.
Tens of thousands have turned out this week for protests nationwide over the government's move to end fuel subsidies, which caused petrol prices to more than double in a country where most people live on less than $2 per day.
At least six people were killed on the first day of protests, including one person allegedly shot by police in Lagos.
Local media reported that three others were killed in southwestern Ogun and Osun states on Tuesday, one by a police officer, but authorities have not confirmed the deaths.
Meanwhile, spiralling ethnic and religious violence in various parts of the country has fuelled further chaos amid warnings of a wider conflict in a country roughly divided between a mainly Muslim north and predominantly Christian south.
Twenty people were killed in four separate incidents in the latest such violence.
The attacks included the shooting dead of four Christian Igbos on Wednesday by suspected members of Islamist group Boko Haram on the outskirts of the northeastern city of Potiskum, according to residents.
Yobe state, where Potiskum is located and which has been hit by repeated violence, also declared a night curfew.
Separately, a police station in the northeastern city of Yola was attacked by unknown gunmen on Wednesday, killing one officer, police said.
Boko Haram has been blamed for scores of attacks, and in recent weeks has claimed responsibility for violence targeting Christians, who have vowed to defend themselves.
A video has emerged on YouTube purporting to show suspected Boko Haram leader Abubakar Shekau sending a message to the president, but the person in the clip appears significantly different from previous images said to be of Shekau.
Amid the sectarian and social turmoil, Nobel literature prize laureate Wole Soyinka, one of the country's most respected voices, warned Tuesday that Nigeria was heading toward civil war.
A statement signed by Nigerian writers, including renowned novelist Chinua Achebe, author of the novel "Things Fall Apart," also warned that the attacks were "precursors to events that could destabilise the entire country."
"Clearly, the sophistication and deadly impact of the terrorist attacks suggest an agenda to create widespread fear and, possibly, to foment anarchy or war," it said.