Sierra Leone President Julius Maada Bio said most of the leaders of an attack on a military barracks in the capital Freetown earlier on Sunday had been arrested, adding that security operations and an investigation were ongoing.

"We will ensure that those responsible are held accountable," Bio said on national television.

"As your commander-in-chief, I want to assure everybody who is resident in Sierra Leone that we have overcome this challenge," he said, and calm had been restored.

Earlier, the government said security forces had repelled "renegade soldiers" who attempted to break into a military armoury in Freetown during the early hours of Sunday.

A nationwide curfew was imposed. Gunfire was heard across the city as the assailants attacked a prison and a police station.

It was not immediately clear if there were any casualties in the barracks attack or during the gunfire in Freetown on Sunday.

The country's former president Ernest Bai Koroma, said in a statement that a military guard assigned to his residence in the capital was shot point blank, while another was "whisked away to an unknown location".

Koroma did not say who shot the guard. He condemned the killing and the attack on the barracks.

"I am deeply concerned that once again our beloved nation could be subject to such insecurity," he said.

The West African country's civil aviation authority urged airlines to reschedule flights after the curfew was declared, while a soldier on its frontier with neighbouring Guinea told Reuters they had been instructed to shut the border.

A Reuters journalist, who earlier witnessed an armed group of men commandeer a police vehicle near the Wilberforce barracks, said streets were mostly empty on Sunday as residents hunkered down.

"We'll clean this society. We know what we are up to. We are not after any ordinary civilians who should go about their normal business," one of the masked men, who was dressed in military fatigues, said before driving away.

Sierra Leone has been tense since Bio was re-elected in June, a result rejected by the main opposition candidate and questioned by international partners including the United States and the European Union.

In August 2022, at least 21 civilians and six police officers were killed in anti-government protests in Sierra Leone, which is still recovering from a 1991-2002 civil war in which more than 50,000 were killed. Bio said the protests were an attempt to overthrow the government.

In his address to the nation on Sunday night, Bio called on Sierra Leone's political and traditional leaders, and civil

society to work to preserve peace.

"Let us not succumb to fear or division," he said.

Information minister Chernor Bah said earlier on Sunday that security forces were making progress in apprehending those involved in the attack, but gave no further details.

A video on social media showed three men, two in fatigues and one in civilian clothes, with their arms tied behind their backs sitting in a military truck surrounded by soldiers. Reuters has not authenticated the video.

Bah said that major detention centres including the Pademba Road prisons were attacked and inmates released by the unidentified assailants.

It was not immediately clear how many prisoners had broken out of the facility, which a U.S. State Department report said was designed for 324 inmates but held more than 2,000 in 2019.

Videos posted on social media, which were not authenticated by Reuters, showed several people fleeing from the area of the prison, while gunshots could be heard in the background.

"The security forces were forced to make a tactical retreat. The prisons were thus overran," said Bah, who had earlier declared a nationwide curfew and called for people to stay indoors.

The Economic Community of West African States condemned what it called an attempt by certain individuals to "acquire arms and disturb constitutional order" in Sierra Leone. The U.S. embassy in Freetown said such actions were not justified.

There have been eight military coups in West and Central Africa since 2020.

Reuters