United Nations troops have shot dead two people as angry mobs stormed the UN base in the Democratic Republic of the Congo capital, Kinshasa.
The deaths came as thousands of angry Congolese targeted the UN over the capture of the eastern town of Bukavu by dissident soldiers.
They say UN peacekeepers should have prevented the rebel advance, which casts doubts on last year’s peace deal.
Meanwhile, the rebel leader has said his fighters will leave Bukavu.
The protests began on Wednesday, when news broke that the town had fallen.
Anti-UN protests have also been reported in Bukavu, Kisangani and Kindu.
DR Congo President Joseph Kabila has accused neighbouring Rwanda of being behind the rebels.
Bukavu is “under the control of Rwandan occupants”, he said, telling the BBC it was “a situation of war”.
Rwanda has denied any involvement.
The UN troops opened fire after mobs broke down the main door at the organisation’s logistical base in Kinshasa, UN mission spokesman Hamadoun Toure said.
“They entered, and there were very many of them,” Mr Toure said. “They began to loot.”
He said the UN troops guarding the building had opened fire “for reasons of legitimate self-defence”.
He added: “We regret this deeply because our mission was to establish peace in the country but we were left with no choice.”
Looting and rape
Latest reports from Bukavu speak of looting and rape, despite efforts by UN peacekeepers to maintain order.
The dissident soldiers are from a former Rwandan-backed rebel group that was supposed to join the national army.
Rebel leader Brig Gen Laurent Nkunda and other renegade commanders – ethnic Banyamulenges, related to Rwandan Tutsis – say government forces have been attacking members of their community.
However, Gen Nkunda said he had already ordered 300 fighters to leave Bukavu and the rest would depart later on Thursday but remain “around” the town.
He told the BBC’s Rob Walker in Bukavu that he did want to control Bukavu, merely to protect the Banyamulenge.
“When the government will appoint a new commander, who will secure my people, I will go back to Goma,” he said.
Goma, a Congolese town north of Bukavu, is controlled by Gen Nkunda’s former rebel RCD under the power-sharing deal agreed last year.
Additional UN peacekeepers were sent to Bukavu last week, following fighting between the dissidents and regular troops.
But the 1,000-strong UN force did not oppose the advancing rebel fighters, saying it did not have a mandate to “make war”.
In the capital Kinshasa, President Kabila on Wednesday said: “It is clearly an attack on our country by Rwandan troops.
“The army is mobilizing with the objective of retaking control of Bukavu.”
The UN said Rwandan troops had been spotted in DR Congo north of Bukavu six weeks ago, although they were all supposed to have left as part of last year’s peace deal.
While Rwanda has denied having any troops in DR Congo, Foreign Minister Charles Muligande said that Rwanda would intervene to oppose “genocide”.
UN officials say that Gen Nkunda has at least 2,000 men under his control – up from 500 just two weeks ago.
They are reinforcing Col Jules Mutebusi, whose militia first clashed with the army last week.
A week of clashes has left at dozens of people dead, and thousands of Banyamulenge have fled into Rwanda.
Both Col Mutebusi and Gen Nkunda are former members of the largest rebel group, the RCD, which controlled all of eastern DR Congo during the five-year war.
Under a peace deal agreed last year, all the Congolese warring factions were supposed to unite as a single army, but progress has been slow.