A boiler exploded as gas leaked into a packed office block.
About 50 people evacuated Martello House in Portland Road, Hove, when they smelt the gas.
Within minutes the three-floor building, which houses Customs and Excise workers, was rocked by a series of explosions.
Fire crews from Roedean and Hove arrived at the scene at about 2.15pm and nine firefighters pumped air into the building to disperse the gas.
Transco turned off the gas supply and examined the boiler.
A spokesman for East Sussex Fire and Rescue Service said: "The occupants of the building did exactly as they should and prevented a very serious incident. Luckily, the explosions did not start a fire."
A stretch of Portland Road was closed for 30 minutes. Fire crews left the scene by 3pm.
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