A day after the rebels marched into the lakeside city of Goma, protesters in the capital Kinshasa attacked a United Nations compound and embassies including those of Rwanda, France and the United States, expressing anger at what they said was foreign interference.
M23 fighters entered Goma on Monday in the worst escalation since 2012 of a three-decade conflict rooted in the long fallout from the Rwandan genocide and control of DR Congo's abundant mineral resources.
The DR Congolese government and the head of UN peacekeeping have said Rwandan troops were present in Goma, backing up their M23 allies.
Rwanda has said it was adopting a defensive posture because of the threat posed to it by DR Congolese militias.
Dozens of DR Congo troops had surrendered but some soldiers and pro-government militiamen were holding out, residents and UN sources said.
People in several neighbourhoods reported small arms fire and some loud explosions on Tuesday morning.
South Africa says three of its UN peacekeepers have been killed in crossfire between troops and M23. (AP PHOTO)
"I have heard the crackle of gunfire from midnight until now ... it is coming from near the airport," an elderly woman in Goma's northern Majengo neighbourhood, close to the airport, told Reuters by phone.
Jens Laerke, spokesman for the UN humanitarian office OCHA, told a briefing in Geneva that colleagues had reported "heavy small arms fire and mortar fire across the city and the presence of many dead bodies in the streets".
"We have reports of rapes committed by fighters, looting of property ... and humanitarian health facilities being hit," he added.
Other international aid officials described hospitals overwhelmed with wounded being treated in hallways.
"The town is a powder keg," Willy Ngumbi, a bishop in Goma, said.
Explosives had hit a house where priests were staying and the maternity ward of a Catholic hospital on Monday, he said by phone.
"The youth are armed and the fighting is now taking place in the town."
The UN and global powers fear the conflict could spiral into a regional war akin to those of 1996-1997 and 1998-2003 that killed millions, mostly from hunger and disease.
Corneille Nangaa, leader of the Congo River Alliance that includes the M23, has suggested the rebels' aim is to replace President Felix Tshisekedi and his government in the capital 1600km.
In Kinshasa, angry crowds burned tyres, chanted anti-Rwanda slogans and attacked diplomatic installations of several countries seen as favourable to Rwanda, leading the police to fire tear gas.
A European diplomatic source said the Rwandan, French, US, Ugandan and Kenyan embassies had been targeted.
"What Rwanda is doing is with the complicity of France, the US and Belgium. The Congolese people are fed up. How many times do we have to die?" said protester Joseph Ngoy.
The United Nations has been caught up in the fighting with a peacekeeping force.
South Africa said three of its peacekeepers were killed in a crossfire between government troops and rebels and a fourth had succumbed to wounds from earlier fighting, bringing the number of its fatalities in the past week to 13.
It said South African President Cyril Ramaphosa had spoken by phone to Rwandan President Paul Kagame, and the two had agreed on the need for a ceasefire.
On Monday, Rwanda's army reported five people killed and 26 injured in exchanges of fire with DR Congolese troops near the border.
The fighting has sent thousands of people streaming out of Goma, a regional hub for humanitarian aid for displaced people.
Hundreds of thousands have fled fighting since the start of the year, on top of three million displaced in eastern DR Congo last year.
DR Congo's government has called on international powers to pressure Rwanda, potentially via sanctions, to end the M23 offensive.
In a phone call with DR Congo President Tshisekedi on Monday, US Secretary of State Marco Rubio "condemned the assault on Goma by the Rwanda-backed M23 and affirmed the United States' respect for the sovereignty of the DRC," the State Department said.
