Nigeria's main city Lagos reeled Wednesday after the shooting of peaceful protesters, with Amnesty International saying security forces had killed several 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.
Amnesty said it was seeking to determine the number of dead.
The Lagos governor said the authorities were investigating the death of one person due to "blunt force trauma to head" and that 25 others were wounded.
The Nigerian army did not respond to AFP's requests for comment but labelled reports of soldiers shooting on protesters as "fake news" on Twitter.
The centre of the city, home to 20 million people, was deserted and shops were closed Wednesday.
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 there were sporadic clashes between bottle-throwing youths and police, who occasionally shot into the air.
Lagos state governor Babajide Sanwo-Olu said in a televised broadcast that no deaths were "recorded" while two of the 25 people wounded were receiving intensive care.
He ordered all "state activities" to be halted across the sprawling city for three days and said he had asked for a probe into "the rules of engagement employed by the men of the Nigerian army that were deployed to Lekki tollgate (the epicentre of the demonstration) last night".
"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 on Twitter.
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.
Anger over abuses by the police's loathed Special Anti-Robbery Squad (SARS) erupted into widespread protests, drawing thousands of people into the streets.
President Muhammadu Buhari was yet to directly address the incident and was expected to hold his regular cabinet meeting at 1000 GMT.
In statement on the overall protests the presidency said Buhari had appealed for "understanding and calm across the nation".
"President Buhari's commitment to extensive police reforms should never be in doubt," it added.
Up until Tuesday some 18 people had died in the demonstrations as clashes were reported between protesters and assailants wearing civilian clothes.
Pictures and videos showing scenes of chaos from the shooting in Lagos were widely shared on social media.
"5am. We're still getting calls from people from Lekki. All through the night. Some hiding near there, some hurt, some just completely panicked," Nigerian podcaster Feyikemi Abudu, who has been actively involved in the protests, said on Twitter.
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."
"The United States must stand with Nigerians who are peacefully demonstrating for police reform and seeking an end to corruption in their democracy," Biden said in a statement.
"I encourage the government to engage in a good-faith dialogue with civil society to address these long-standing grievances and work together for a more just and inclusive Nigeria."
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.
Since Tuesday, Rihanna, Beyonce, Bobi Wine and Manchester United's Odion Ighalo added their names to a list of celebrities who have supported the protesters.