HAZMAT crews wearing gas masks are at the Harbour Hotel on Kings Road after reports of “smoke and a gas alarm sounding”
An East Sussex Fire and Rescue Service spokesperson said: "At 12.30pm we were called to an alarm sounding at a hotel in Kings Road, Brighton.
"The premises have been evacuated. Firefighters are still at the scene. No casualties reported."
Part of the seafront road is sealed off for what is believed to be a chemical leak.
A fire service spokesperson added: "Brighton seafront from Madeira Drive to West Street (is closed), due to an ongoing incident.
"Part of the road between Middle Street and West Street is closed. Drivers urged to use an alternative route."
A Twitter user wrote: "Chaos on Brighton Seafront, possible chemical or gas leak at the Harbour hotel, bottom of West Street.
"Special technical rescue teams and equipment turning up."
There are huge delays by the seafront with a cordon in place around the hotel at the bottom of West Street.
There is traffic queuing up West Street into Queens Road and along Kings Road itself.
Buses have been diverted and it has led to congestion across the town.
A police spokesman said: “We are assisting fire service with an incident that they are leading on.
"We are providing help with road closures.”