The curfew was due to start at 3 p.m. local time (1500 GMT) in a bid to stem the violence.
Videos on social media showed large crowds of protesters and piles of burning tyres in eastern Freetown. Other footage showed a group of young men throwing rocks on a street filled with whitish smoke and another group attacking a man on the ground. Reuters could not independently verify the images.
"These unscrupulous individuals have embarked on a violent and unauthorised protest which has led to the loss of lives of innocent Sierra Leoneans including security personnel," said Vice-President Mohamed Juldeh Jalloh in a video address.
He did not say how many people had been killed. It was not immediately possible to reach the police for comment.
"The government hereby declares a nationwide curfew," he said. "The security sector has been authorised to fully enforce this directive."
Earlier, internet observatory NetBlocks said Sierra Leone faced a near-total internet shutdown with national connectivity at 5% of ordinary levels.
On Tuesday, the national security coordinator asked the armed forces to be prepared to back up the police from Aug. 9 to 12, warning of a "potentially volatile security situation", according to an internal letter shared widely online.
Long-standing frustration with the government in some quarters has been exacerbated by rising prices for basic goods in the West Africa country, where more than half of its population of around 8 million live below the poverty line, according to the World Bank.
(Writing by Alessandra Prentice; Editing by Toby Chopra and Mike Harrison)
By Cooper Inveen
