Liberia's President Gyude Bryant has imposed an immediate curfew in the capital Monrovia, after heavy fighting spread through the city overnight.
Disarmament is due to be completed by the end of October
Eyewitnesses say a religious dispute on Thursday evening mushroomed into riots, destroying five churches and a mosque.
United Nations peacekeepers struggled to restore calm as rampaging groups looted into the morning.
Mr Bryant heads a transitional power-sharing government set up to organise elections after 14 years of civil war.
"The curfew starts now," Mr Bryant said in a statement on local radio on Friday morning.
"The United Nations mission has been instructed to use every force to put the situation under control."
The BBC's Jonathan Paye-Layleh in Monrovia says sporadic shooting can still be heard.
Speaking on the UN radio station, UN Police Commander Mark Kroeker says several houses were destroyed and numerous people injured.
"We have had to extract numerous people from
various situations and rescue them from mobs during the night," he said.
Eyewitnesses say armed ex-combatants were involved in the fighting, but the authorities say any shots fired were by the UN, our correspondent says.
There are some 15,000 UN peacekeepers in Liberia - they are due to finish disarming the estimated 40,000 former militiamen by the end of October.
It is not clear what caused the violence in the commercial suburb of Paynesville, which spread across the capital to other districts including the port area.
Christian and Muslim leaders appealed for calm on local radio and said they were launching an investigation.
UN peacekeepers have moved into the commercial district of Duela to secure the area against further looting, our correspondent says.
Monrovia has largely been calm since the former President Charles Taylor went into exile in August last year as part of a peace deal to end the civil war.