The government said those attempting to break into the armoury had been repelled. Witnesses told media they heard gunshots and explosions in the city's Wilberforce district.
The information ministry said in a statement that it assured the public that "the government and our state security forces are in control".
"To enable the security forces to continue the process of apprehending the suspects, a nationwide curfew is declared with immediate effect across the country," the ministry said.
No further details were given on the alleged perpetrators of the attack, or their motives. Sierra Leone, an English-speaking country in West Africa, has been going through a political crisis following presidential and general elections in June this year.
The Economic Community of West African States (ECOWAS) issued a statement underlining "its zero-tolerance for unconstitutional change of government".
Echoing language used to condemn past coup attempts, ECOWAS spoke of its "utter disgust" over a "plot by certain individuals to acquire arms and disturb the peace and constitutional order in Sierra Leone".