Fresh unrest in Nigeria's Lagos after shooting of protesters

Shots rang out and a prison was set ablaze as fresh unrest rocked Nigeria's biggest city Lagos on Thursday after the shooting of protesters that drew international outrage.

Gunfire was heard and smoke could be seen billowing from the detention facility in the upscale Ikoyi neighbourhood in central Lagos, an AFP journalist said.

Police said assailants had attacked the site on the second day of violence in the city of 20 million people after a brutal crackdown by security forces on demonstrations.

Amnesty International said Nigerian soldiers and police gunned down 12 peaceful protesters on Tuesday, while 56 have died overall across the country since demonstrations against police abuse and bad governance began on October 8.

The United States on Thursday condemned what it called excessive use of force by the Nigerian military.

"We welcome an immediate investigation into any use of excessive force by members of the security forces," Secretary of State Mike Pompeo said in a statement.

"Those involved should be held to account in accordance with Nigerian law," he added.

President Muhammadu Buhari, who has not yet commented on the shooting, was holding a national security council meeting on Thursday with his defence minister and chief of police, according to a statement.

Vice President Yemi Osinbajo said his "heart goes out" to the victims of the shooting as well as policemen and others who have lost their lives in several days of turmoil in Africa's most populous country.

"The pain of these terrible events is palpable in our towns and cities, and some losses are irreplaceable, but we can and will get justice for all of them," a statement said.

Nigeria's army has dismissed as "fake news" reports that soldiers opened fire on demonstrators.

Police Minister Muhammad Maigari Dingyadi told the BBC that troops were not ordered to open fire on protesters.

"I cannot say who is involved in the shooting... definitely not the police. Soldiers have already spoken about this, they are denying their involvement," he said.

Pictures and videos from the attack were widely shared on social media, unleashing a wave of anger at the authorities.

The governor of Lagos announced a round-the-clock curfew and a pause on state activities until Friday.

Soldiers patrolled the largely deserted streets of the city on Thursday a day after buildings including police stations, a TV channel and the port were torched.

An AFP journalist saw the wreckage of a dozen cars still smouldering in a middle-class neighbourhood and two official buildings had been burnt down.

Demonstrations erupted two weeks ago in Nigeria over brutality by the police's loathed Special Anti-Robbery Squad (SARS).

The unit was disbanded but protests quickly spiralled into broader demands for change as promises of reform failed to stem the anger and violence flared.

The African Union on Thursday strongly condemned the violence and called on all parties to "privilege dialogue".

The United Nations, European Union and Britain have all called for those responsible for the killings to be held responsible.

Protests have been held in South Africa and London over the shooting, with a string of major celebrities throwing their weight behind the calls for justice.