A rebel coalition captured Bangassou, a south-eastern city in the Central African Republic, United Nations officials have said.
Rebel forces staged a dawn raid and were now everywhere in Bangassou. The rebel coalition currently controls two thirds of the country.
They accuse the government of holding a fraudulent election on 27 December.
The government has denied allegations of poll rigging. Initial results of the election for president and seats in the national assembly are expected on Monday.
The authorities have also accused former President François Bozizé, who supports the rebels of trying to stage a coup, which he denied.
The Central African Republic (CAR) has seen several coups since independence from France 60 years ago.
CAR is one of Africa’s poorest countries, even though it is rich in resources like diamonds and uranium. The UN, which has nearly 13,000 peacekeepers on the ground, estimates that half of the population are dependent on humanitarian assistance.