INVESTIGATORS do not believe a mysterious anarchist group is responsible for a fire that badly damaged a luxury car dealership in West Melbourne last night.

The group posted a statement online, on a website that the Herald Sun has chosen not to name, which said activists had broken into Gran Turismo Autos on Spencer St last night and started the fire.

"The flames of anarchist insurrection roared high above the toxic skies of Melbourne," the group's statement said.

But the Herald Sun understands police arson and explosives squad experts and MFB fire investigators have analysed and discounted that theory.

Det-Sen-Sgt Jeff Maher said he was aware of the group's claims.

"But at this point in time the cause of the fire is undetermined," he said.

"We should have some sort of result within the next day ... The premises is about to be handed back to the owners."

It is believed the fire may have been started by a cigarette but it is unclear if the fire was an accident or deliberately lit.

About a dozen cars in the showroom were damaged in the fire, which caused hundreds of thousands of dollars damage.

An apparel business next door escaped the blaze but its store was flooded and their stock was damaged.

Solomon Brothers Apparel's Andrew Cohen said about $150,000 of stock had been rendered useless by smoke and water.

But he said they were lucky their whole business had not been destroyed in the blaze.

No one was injured and MFB spokesman Norm Henderson said about 10 people had been inside and evacuated themselves.

About 50 firefighters battled the blaze, which is believed to have caused hundreds of thousands of dollars in damage.

Crews were called to the fire about 8.30pm and were still pumping water to the two-storey concrete and steel building more than two hours later.

Mr Henderson said the fire was contained to an office section in the building’s top floor, which was filled with files, papers and tyres.

Firefighters had difficulty accessing the area due to heavy smoke and the structural integrity of a rear wall.

A huge cloud of black smoke covered the block last night, as two ladder platforms poured water on the roof while hoses attacked the flames from multiple angles.

Dozens of curious passers-by stood around to watch.