Fighters from the M23 group entered Goma following days of clashes with UN-backed Congolese soldiers that forced tens of thousands of residents to flee. A senior UN source said inter-national peacekeepers had given up defending the city after the Congolese troops evacuated.
The rebellion has aggravated tensions between Congo and its neighbour Rwanda, which Kinshasa's government says is orchestrating the insurgency as a means of grabbing the chaotic region's mineral wealth. Rwanda denies the assertion.
Witnesses described heavily armed rebels walking through the streets of Goma, near Rwanda's border on the edge of Lake Kivu, past UN armoured vehicles and peacekeepers.
"Despite the attack helicopters, despite the heavy weapons, the FARDC (Congo national army) has let the town fall into our hands," Colonel Vianney Kazarama, a spokesman for M23, said.
He said the rebels had left a corridor open for Congolese forces to evacuate.
Goma's capture will be an embarrassment for President Joseph Kabila, who won re- election last year in polls that triggered widespread riots in Kinshasa and which inter-national observers said were marred by fraud.
The senior UN source said international peacekeepers had been unable to mount a defence after Congolese troops evacuated.
"There is no army left in the town, not a soul ... once they were in the town what could we do?"