DR Congo's Mount Nyiragongo has erupted for the first time in nearly two decades.
Many thousands of people have fled the city of Goma, some heading for its highest point Mount Goma.
Others headed for the nearby Rwandan border, with authorities there saying that around 3,000 people had already crossed over on Saturday night.
Goma resident Zacharie Paluku told The Associated Press: "Everyone is afraid, people are running away. We really don't know what to do."
There was a government evacuation plan but it came hours after the sky had turned a fiery red and power had been cut to the city.
Lava from the eruption flowed onto a major road on Saturday as it reached the airport on the city's edge, according to a volcanologist based there.
Dario Tedesco told Reuters that new fractures were opening in Nyiragongo, allowing the lava to flow southward towards Goma after initially flowing east toward Rwanda.
"Now Goma is the target," Mr Tedesco said, adding: "It's similar to 2002."
Nyiragongo's last eruption was in 2002 and it left hundreds of people dead, and coated airport runways with lava.
"I think that the lava is going towards the city centre," Mr Tedesco said.
"It might stop before or go on. It's difficult to forecast."
No casualties have been confirmed as a result of the eruption so far.
Goma is a base for UN peacekeeping mission MONUSCO and it was reported that the organisation's aircraft had been moved to nearby cities.
There had been concern about the amount of volcanic activity observed in the last five years at Nyiragongo but experts at Goma Volcano Observatory have not been able to check things regularly due to funding issues.