At one point flames poured from windows at both sides of the building.
The three-storey building, at the junction of Kirkstall Road and Wellington Street, has housed regional staff of the old National Association of Local Government Officers, Royal Mail, and a law firm.

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The offices have been out of use and boarded up for some years.
The alarm was raised at 1.20am today and two fire tenders raced to the building from Leeds Central fire station in Kirkstall Road.

Back-up was called for and two tenders from Hunslet fire station turned out and an aerial appliance was brought in.
Leeds Central Crew Manager Darrell Hird said: "The fire was on the second floor. Most of the ceilings came down."

Fire crews believed the building may have been used by people living rough but there had been no one trapped in the fire.

The cause is being in treated as suspicious.

It is believed the block is due for demolition to be replaced by flats.