"A curfew is imposed on all persons throughout the Kingdom of Lesotho with effect from today," read a government gazette published Tuesday.
Locals will not be allowed out after 2000 GMT, according to the document signed by police commissioner Holomo Molibeli.
The curfew is to remain in place until further notice, Molibeli said. Those failing to comply face a fine or up to two years in jail.
Police Minister Lebona Lephema said the move was aimed at tackling gun violence across the country.
The move comes days after a prominent radio journalist was shot dead in a killing that shocked the nation.
Journalist Ralikonelo Joki, host of a current affairs show on a local private radio, was gunned down by unknown assailants as he was leaving his workplace in Maseru on Sunday night.
No arrests have been made in connection with the case, and investigations are ongoing.
The Committee to Protect Journalists, a media advocacy group, has urged authorities to launch "a credible investigation" into the killing, adding Joki was known for his work on corruption.
Gang related shootings are common in Lesotho, a landlocked mountainous country of two million people, where police complain the number of unlicensed firearms in circulation is too high.