A Northampton town centre street has been closed off by police.
Witnesses said it looked like a manhole cover had 'exploded' in Gold Street at about 12.30pm today.
Northamptonshire Police said Gold Street itself has been closed to traffic and pedestrians. At this stage, police believe one person has been injured.
A spokeswoman said: "Access around the immediate area is limited."
Onewitness, Nicola, was standing behind the cordon after her and her colleagues were evacuated from Poundstretchers. "We heard a loud boom and saw flames from the gas main.
"A man who works in the butcher's came out with a fire extinguisher but wasn't able to put it out.
"It sounded like an explosion of gas canisters.
"I looked outside and saw the flames coming out of the road, and black smoke from where they were trying to put it out."