The gas leak was reported at 12.21am at a block of flats in Milton Mount Avenue, Crawley. Around 700 people have been evacuated and placed in temporary accommodation.
The response included personnel from West Sussex Fire and Rescue Service, South East Coast Ambulance Service, West Sussex County Council, Crawley Borough Council and Southern Gas.
Sussex Police detective chief superintendent Steve Rayland said: “This is an ongoing incident which has caused huge disruption to hundreds of people on a cold January morning, and I’d like to thank everyone for their patience and understanding as we continue to work with our partners to minimise the ongoing disruption.
“We are working hard to ensure this is dealt with as soon as possible to enable everyone to return home, however this may take several hours yet.
“In the meantime, we are urging everyone to stay away from the area to allow the responding agencies to focus on resolving the incident to allow residents to return home.
“The exact cause of the gas leak is yet to be determined but not believed to be criminal, and we will provide more information as soon as we are able to.”
Resident Leah Hicklin said she was evacuated at 1.30am but others were still leaving the building at 4am.
She told the BBC: “We thought it was only something minor until they started evacuating the bottom floors and then I was the top floor and I could smell gas.
“So they slowly but surely evacuated every single one of us.
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Dan Brown, from gas company SGN, said a broken pipe had been identified as the source of the leak and repairs have been made by engineers.
Mr Brown said: “We remain on site this morning carrying out further safety checks and monitoring gas level readings inside the building.
“When the building has been fully ventilated and there are no readings of gas, we’ll be able to safely allow residents to return to their properties.
“All being well, we hope to be able to do this later today.”
No injuries have been reported.
Some of the residents stayed with family or friends overnight, while others were placed in hotels.
Local ward councillor Tina Belben saida rescue centre could not be set up due to coronavirus restrictions.
