UN envoy Jacques Klein said the deaths and injuries, which left more than 200 people in hospital, were caused by clubbing, beatings and machete wounds.
The violence is the worst seen in the city since former president Charles Taylor was forced into exile.
Following the passing of a disarmament deadline on Sunday, UN peacekeepers are conducting door-to-door searches for weapons, and attempting to reimpose their authority.
Some 95,000 fighters have handed over weapons in the UN programme.
