Bangladeshi United Nations peacekeepers have pulled out of a camp in western Ivory Coast after clashing with supporters of President Laurent Gbagbo.
Ivorian forces have done little to prevent street protests
Four of the protesters were killed after a UN base in Guiglo, 300km (187 miles) from Abidjan, was attacked.
After three days of protests the crisis is escalating, with youths surrounding UN and French buildings in Abidjan, the main city, a BBC correspondent says.
Gbagbo supporters are angry at attempts to dissolve parliament.
International mediators this week recommended that parliament, whose mandate has expired, be discontinued.
IVORY COAST CONFLICT
2002, Sept: Dissident soldiers fail to overthrow President Gbagbo, but rebels seize north of country
2003, May: Armed forces sign ceasefire with rebel groups
2004, Nov: Ivorian air force attacks rebels; French forces destroy parts of Ivorian air force after nine of their soldiers killed. Violent anti-French protests prompt thousands of Westerners to leave
2005, Oct: Planned elections shelved as President Gbagbo invokes a law which he says allows him to stay in power
The BBC's James Copnall, in Abidjan, says the national assembly is one of Mr Gbagbo's last power bases in divided Ivory Coast.
Following the mediators' move, the ruling party pulled out of the transitional government and UN-backed peace talks, and called on the 10,000 French and UN peacekeepers the peace to leave.
The country has been split in two since a failed coup attempt in 2002. Ivorian rebels, who control the north, say the country is on the brink of war.
Nigerian President Olusegun Obasanjo is due to arrive in Ivory Coast on Wednesday for talks with Mr Gbagbo about the latest crisis.
In the early hours of Wednesday, the UN base at Guiglo was besieged by youths supporting President Gbagbo, known as "Young Patriots".
The 300-strong Bangladeshi peacekeeping force stationed there responded with force.
"They had to defend themselves," said Captain Gilles Combarieu, a UN military observer.
"I know there are four from among the attackers [who were killed]," UN spokeswoman Margherita Amodeo told Reuters.
The UN decided to abandon the base and the peacekeepers were being withdrawn to the demilitarised zone further north, Capt Combarieu said.
Our correspondent says the Young Patriots are also now in control of virtually all the main streets in Abidjan.
For a third day, youths are stopping cars and checking identity papers at roadblocks. There is little public transport , and debris from Tuesday's unrest is strewn across roads.
Despite a ban on street demonstrations, the Ivorian security forces have not acted to stop the protests. There have been few violent incidents but Abidjan and several other cities are paralysed.
France expressed concern and called for calm in Ivory Coast, and the head of French defence staff called for sanctions against its former colony.
International mediators were appointed by the UN to help steer the country towards elections, due this year.
But the ruling Ivorian Popular Front (FPI) accused the international community of carrying out a "constitutional coup d'etat".
A spokesman for the rebel New Forces, which hold the north, responded: "There is not a future for Ivory Coast if the FPI succeeds in making a putsch against the peace process. That means war."
Our correspondent says the crisis is the biggest test faced by Prime Minister Charles Konan Banny since his appointment last month.
Analysts say by calling for the dissolution of parliament, international mediators intended to strengthen Mr Banny's authority and ensure that hostile deputies did not block attempts to implement the peace process, as happened last year.
The elections had been due last October but had to be postponed because of the continued instability.