The inferno destroyed the dilapidated and vacant Kothari Building in Fort area of Central Business District around 4.00 a.m.
As a team of firemen rushed to fight the conflagration, the blaze spread and came in contact with some old electrical appliances and fittings lying there.
The firemen were caught unawares when the south portion of the structure suddenly collapsed around 6.45 a.m.
A fire brigade tanker and a specialised equipment unit were also damaged in the crash.
The Brihanmumbai Municipal Corporation (BMC) Disaster Cell said the building was abandoned more than five years ago after its main staircase crashed, rendering it unfit for habitation.
The fire-fighters took precaution to control the blaze from outside with the water curtain and not allow it to spread to adjoining buildings in the congested area.