A number of people have reportedly been shot dead or wounded at a protest against police brutality in Nigeria's biggest city, Lagos.
Witnesses and the rights group Amnesty International said several people were killed and wounded when soldiers opened fire.
The state governor said about 25 people had been wounded but only one person had died.
An indefinite 24-hour curfew has been imposed on Lagos and other regions.
Protests over a now-disbanded police unit - the Special Anti-Robbery Squad (Sars) - have been taking place for two weeks, with demonstrators using the social media hashtag #EndSars to rally crowds.
As tensions continued to rise on Wednesday, police in different districts of Lagos fired shots in the air to disperse people defying the curfew, the BBC's Nduka Orjinmo reports from the capital, Abuja.
Protesters who briefly gathered again at the Lekki toll gate - where Tuesday's shootings took place - were forced out by police, he adds.
President Muhammadu Buhari has appealed for "understanding and calm".
However, plumes of smoke could be seen above Lagos on Wednesday.
A reporter for AFP news agency said several buildings were in flames around the Lekki area. A bus station was reported to be on fire in another district while a major TV station - linked to one of the governing party's top politicians - was reportedly set alight by people throwing petrol bombs.
The headquarters of the Nigerian Ports Authority was also set on fire, local media said.
What do we know about the shooting?
Witnesses spoke of uniformed men opening fire in the wealthy Lekki suburb on Tuesday evening.
Soldiers were seen barricading the protest site moments before the shooting, BBC Nigeria correspondent Mayeni Jones reports.
Social media footage streamed live from the scene shows protesters tending to the wounded.
An unnamed witness told BBC News that shortly before 19:00 local time soldiers "pulled up... and they started firing directly" at peaceful protesters.
"They were firing and they were advancing straight at us. It was chaos. Somebody got hit straight beside me and he died on the spot," he said.
Four witnesses told Reuters news agency that soldiers had opened fire on demonstrators. One of them, Alfred Ononugbo, 55, said: "They were firing into the crowd. I saw the bullet hit one or two persons."
The Premium Times newspaper quoted witnesses as saying about 12 people had been killed.
In a tweet, Amnesty International Nigeria said it had "received credible but disturbing evidence of excessive use of force occasioning deaths of protesters at Lekki toll gate in Lagos".
How have the authorities reacted?
In a statement on Wednesday, President Buhari did not directly refer to the shootings, but called on people to have patience as police reforms "gather pace".
A statement issued by his office said the dissolution of the Sars was "the first step in a set of reform policies that will deliver a police system accountable to the Nigerian people".
"The presidency wishes to reiterate the full commitment of the Buhari administration to the implementation of lasting police reforms in Nigeria," it added.
The army has not issued a statement on events in Lekki, but in several Twitter posts it described media reports as "fake news".
Lagos State Governor Babajide Sanwo-Olu, who visited hospitals on Wednesday, said about 25 people had been wounded in what he described as an "unfortunate shooting incident".
He initially said no-one had been killed but later said one person had died in hospital due to "blunt force trauma to the head".
Contradicting the army, he told the BBC's Newshour programme that the military had been at the scene at the time of the shooting despite public assurances that soldiers would not be deployed until after the start of a curfew at 21:00.
"I think about seven o'clock or thereabouts there was a small unit of the military that went [to Lekki] and we heard that gunshots were fired," he said.
What other reaction has there been?
And former US Vice-President Joe Biden - who is standing against President Donald Trump in next month's election - also urged authorities to end the "violent crackdown on protesters".
"The US must stand with Nigerians who are peacefully demonstrating for police reform and seeking an end to corruption in their democracy," he said in a statement.
Nigerian footballer Odion Jude Ighalo, who plays for Manchester United, accused the Nigerian government of killing its own citizens. "I'm ashamed of this government," he said in a video posted on Twitter.
Nigeria reacts to harrowing scenes
Analysis by Mayeni Jones, BBC Nigeria Correspondent
It was a bleak night in Nigeria as social media footage from the shooting poured in, showing gunshots ringing out at the protest site long into the night.
This is not the first time the Nigerian army has been accused of shooting unarmed protesters. There have been reports of violent crackdowns on EndSars protesters in other parts of the country.
But seeing live rounds used at one of the protest sites that had been peaceful until last night has rattled many. Just last week I stood at the very site of the shooting.
The protesters were peaceful, organised, hopeful for the future of their country. But this is no more.
Harrowing social media videos showing protesters singing the national anthem as shots ring out in the background have caused outrage.
Multiple online accounts say the CCTV and lights were taken out at the toll gate where the protest took place before troops started advancing, leading to total chaos.
These details are galvanising a generation already disillusioned with the ruling class. The Nigerian government is running out of time to quell the growing dissatisfaction.
How did the unrest begin?
Protests began nearly two weeks ago with calls for the Sars police unit, which had been accused of illegal detentions, assaults and shootings, to be disbanded.
President Buhari dissolved the unit on 11 October.
But the demonstrators called for more changes in the security forces as well as reforms to the way the country is run.
Mr Sanwo-Olu has said that criminals have hijacked the protests.