The Nigerian army and police killed at least 12 peaceful protesters in two locations of Lagos in a deadly crackdown on demonstrations, according to Amnesty International.

Buildings in Nigeria's main city of Lagos were torched and sporadic clashes erupted after the shooting of the protesters.

Witnesses said gunmen opened fire on a crowd of over 1,000 people yesterday evening to disperse them after a curfew was imposed to end spiralling protests over police brutality and deep-rooted social grievances.

"Evidence gathered from eyewitnesses, video footage and hospital reports confirm that between 6:45 pm and 9:00 pm Tuesday 20 October, the Nigerian military opened fire on thousands of people who were peacefully calling for good governance and an end to police brutality," Amnesty International said.

The shooting has drawn international condemnation, while Nigerian President Muhammadu Buhari has appealed for calm, without directly addressing yesterday's incident.

The Lagos governor at first insisted no fatalities had been recorded but later said the authorities were investigating the death of one person due to "blunt force trauma to the head". 

He said at least 25 others were wounded.

The Nigerian army did not respond to AFP's requests for comment but on Twitter it labelled reports of soldiers shooting on protesters as "fake news".

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Read: Two weeks of spiralling protests in Nigeria

The centre of Lagos, a sprawling city that is home to 20 million people, was deserted and shops were closed Wednesday as a curfew was in force.

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 alight 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," Mr Sanwo-Olu wrote earlier on Twitter.

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 yesterday, some 18 people had died in the demonstrations as clashes were reported between protesters and assailants wearing civilian clothes.

Mr 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 Buhari was yet to directly address the incident but in a statement on the overall 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.

Pictures and videos showing scenes of chaos from yesterday's shooting in Lagos were widely shared on social media.

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".

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 yesterday, 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.