Eyewitness Kahokolo Kambale told Reuters that 50 to 60 people were inside the petrol station. "Nobody could have escaped," he said.
Goma, in the east of the Democratic Republic of Congo, has been devastated by the eruption of Mount Nyiragongo, which has sent lava flowing through the town.
Despite continuing earth tremors and other hazards facing the town, displaced residents have been streaming back in their hundreds from temporary camps across the border in Rwanda to see what remains of their homes.
"I want to return home because my house is still there and I have heard that the lava has stopped," Augustin Mirenge, a Goma school teacher told the Associated Press.
But United Nations officials say that apart from the immediate hazards, there is no fresh water or electricity in Goma, and food is in short supply.
And although an international operation is now gathering pace, so far little aid has reached the area.
The petrol station inferno came after a night of violent earth tremors that could be felt on both sides of the Congo-Rwanda border.
Inaccessible
Fuel exploded shortly before 0900 (0700 GMT), sending a great ball of orange flame shooting into the air. Dense black smoke could be seen from miles away.
The petrol station had apparently been covered with some of the volcanic lava which swept down from Mount Nyiragongo last week and obliterated large parts of Goma.
Looters were using plastic containers to retrieve the fuel and carry it back.
The station is located by one of the main lava flows, but the lava had cooled enough for people to walk across it.
The eruption itself, which sent hundreds of thousands of Goma residents fleeing to Rwanda on Thursday, has killed about 45 people, according to UN estimates.
"The weather is so cold, we can't go to [the camp]... it's in a forest, so the conditions are harsh and there are many mosquitoes," he said.
Reports said the petrol station was still ablaze and inaccessible to rescuers several hours after the blast.
Other tanks were said to have exploded after the initial blast.
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