Crowds were seen with mattresses and other belongings, fleeing towards the border with Rwanda to the east, even before the government's announcement, which came several hours after the eruption started.
The last time Nyiragongo erupted was January 17, 2002, killing more than a hundred people and covering nearly all of the eastern part of Goma with lava, including half of the airport's landing strip.
The lava lake of Nyiragongo volcano is constantly boiling and is usually one of the tourist attractions in the area, not far from Virunga Park where a large number of gorillas live, among many other animal species.
The account also tweeted photos of people arriving in Rubavu district in Rwanda, adding that they would be "accommodated in schools and places of worship that have been made ready".
Volcanologist Charles Balagizi said the observatory's report was based on the direction in which the lava appeared to be flowing, which was toward Rwanda rather than Goma.
"I can see high lava fountains", Tedesco told Reuters. "It's similar to 2002". Smoke rose from smoldering heaps of lava in the Buhene area near the city. "It's hard to forecast", he said. He said lava had reached the worldwide airport on the eastern edge of the city but that it was not likely to reach other parts of Goma. "There is a risk that the lava will flow on Goma", another said. The power was also out across much of Goma.
On Saturday, Communications Minister Patrick Muyaya had said that the government had activated an evacuation plan and was "discussing the urgent measures to take at present".
Congolese President Felix Tshisekedi had said he would "interrupt his stay in Europe to return home this Sunday to supervise the coordination of aid".
On the streets of Goma, panic spread quickly.
"We are panicked because we have just seen the entire city covered by a light that is not electricity or lamps", said John Kilosho. "We really don't know what to do".
Others fled to the city centre from villages and neighbourhoods threatened by lava on the northern outskirts.
"We looked at the sky and saw the red colour of the volcano", said Richard Hazika Diouf from the Majengo neighbourhood.
The previous eruption of Nyiragongo volcano, which is over 3000 meters high, dates back to January 17, 2002.