Buildings in Nigeria's main city of Lagos were torched on Wednesday and sporadic clashes erupted after the shooting of peaceful protesters in which Amnesty International said security forces had killed 12 people.
Witnesses said gunmen opened fire on a crowd of over 1 000 people on Tuesday evening to disperse them after a curfew was imposed to end spiralling protests over police brutality and deep-rooted social grievances.
"We were all sitting down, peacefully, and they shut down the lights and the billboards, everyone started screaming," a protester called Toye told AFP, asking that her full name not be used.
"They came to us, but I don't know who it was. They were shooting, and everyone was running for his life."
Pictures and videos showing scenes of chaos in the aftermath of the shooting were widely shared on social media.
The shooting has drawn international condemnation, while Nigerian President Muhammadu Buhari has appealed for calm, without directly addressing Tuesday's incident.
In South Africa, the continent's other economic powerhouse, hundreds of people took to the streets on Wednesday to voice their outrage at the shooting.
The Lagos governor at first insisted no fatalities had been recorded but later said the authorities were investigating the death of one person resulting from "blunt force trauma to the head".
He said at least 25 others were wounded.
Human Rights Watch said in a statement on Wednesday that the Nigerian army had opened fire on the crowd in "a shooting spree".
"The authorities should immediately withdraw the military from the streets, and identify and prosecute officers responsible," said Anietie Ewang, a Nigeria researcher with the rights group.
The Nigerian army did not respond to AFP's requests for comment but it called reports of soldiers firing on protesters "fake news".
Amnesty said it was seeking to determine the number of dead.
Toughest night of our lives
The centre of Lagos, a sprawling city that is home to 20 million people, was deserted and shops were closed Wednesday under the curfew.
An AFP journalist said several buildings were in flames around the area of the shooting and army patrols could be seen in the street.
In another district a bus station was ablaze and there were sporadic clashes between bottle-throwing youths and police, who occasionally shot into the air.
A major Nigerian TV station linked to one of the ruling party's top politicians was an "inferno" after its director said it was attacked by hoodlums with Molotov cocktails.
Lagos state governor Babajide Sanwo-Olu, in a televised broadcast, ordered all "state activities" to be halted across the city for three days.
He said he had asked for a probe into "the rules of engagement employed by the men of the Nigerian army that were deployed" at the scene during the shooting on Tuesday.
"This is the toughest night of our lives as forces beyond our direct control have moved to make dark notes in our history," Sanwo-Olu wrote earlier.
The United Nations said it decried "the violent escalation on 20 October in Lagos which resulted in multiple deaths and caused many injuries".
Police brutality and abuses
Up until Tuesday some 18 people had died in the demonstrations as clashes were reported between protesters and assailants wearing civilian clothes.
Sanwo-Olu had announced an indefinite curfew from Tuesday afternoon in Africa's largest city after claiming that criminals had hijacked the wave of demonstrations that erupted two weeks ago across the country.
The police chief also ordered anti-riot units to be deployed around the country.
President Buhari was yet to directly address Tuesday's incident but in a statement on the protests the presidency said he had appealed for "understanding and calm across the nation".
"President Buhari's commitment to extensive police reforms should never be in doubt," it added.
Reacting to the shooting, the minority caucus in Nigeria's House of Representatives condemned "such wicked acts of extra-judicial killing of defenceless citizens".
The lawmakers called on the president to "immediately order the arrest and prosecution of officers involved in the Lekki killing".
Nigeria, where the median age is 18, is a tinderbox of profound economic and social grievances, and the demonstrations have snowballed from anger over police violence to broader demands.
UN chief Antonio Guterres called for an "end to reported police brutality and abuses" and called on the authorities "to investigate these incidents and hold the perpetrators accountable", a spokesman said.
The European Union's foreign policy chief Josep Borrell said it was "alarming to learn that several people have been killed and injured during the ongoing protests".
US presidential candidate Joe Biden urged the president and military "to cease the violent crackdown on protesters in Nigeria, which has already resulted in several deaths".
Since Tuesday, Rihanna, Beyonce, Ugandan pop star-turned politician Bobi Wine and Manchester United striker Odion Ighalo added their names to a list of celebrities who have supported the protesters.