Gunfire was heard ringing out across Freetown, as assailants targeted a military barracks, a prison and other locations.
An army spokesperson told Reuters that the 20 dead included 13 soldiers, three assailants, a police officer, a civilian and someone working in private security.
The government has since reduced an all-day curfew to a nightly one, and police were checking vehicles at checkpoints.
Shops and businesses started to reopen on Monday afternoon.
But the fear among residents remains.
This woman says it was the first time she had heard a gunshot and that she hid under her bed.
The government blamed the attacks on "renegade soldiers" and this resident recalled encountering them:
"Military vehicles approached us and some of the officers said we should shout forcefully, 'President Maada Bio must go!' Some of us wanted to answer and some refused to shout. We asked them for which reason we should say that, and he responded by asking if we had the ability to buy expensive bags of rice."
Hours after the rampage, President Julius Maada Bio assured citizens calm had been restored, and that the attackers would be held to account.
Last year, anti-government protests erupted over soaring living costs and at least 21 civilians were killed in the unrest.
Sierra Leone, which is still recovering from a 1991-2002 civil war in which more than 50,000 were killed, has been tense since Bio was re-elected in June.
