Liberia: 250 people arrested after riots in Monrovia
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.
Monrovia is now calm and UN helicopters are hovering overhead.
This has been the worst violence seen 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.
Mr Klein said there were several flash points which were used by former combatants, especially those linked to Mr Taylor, to try to destabilise the country.
"What we are seeing are the death throes of the [old] regime," he commented .
Areas on the outskirts of Monrovia remain tense. Several churches and mosques were attacked during the clashes which began last Thursday, and a curfew has now been put in place.