A fire that tore through a residential and office building in Calcutta has killed 24 people, including two who leapt to their deaths.

Initial reports said six people had died in the blaze, but firefighters later found 18 bodies inside the seven-story structure.

Four others who suffered burn injuries died in the hospital, state home secretary Ardhendu Sen said.

A further 20 people were being treated at the hospital, with five said to be in a critical condition.

Large crowds in the eastern Indian city gathered to watch the rescue operation at the nearly century-old building.

As thick smoke billowed from the fourth floor, officials scrambled to get dozens of people to safety.

It took some 100 firefighters several hours to bring the blaze under control.

An investigation into the cause of the fire was under way, but it was believed to have started in an elevator, police officer Javed Shamim said.

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