Her final text contained just a single word, but it haunts Jean Hanlon's (pictured) family to this day. 'Help', the message read.

A group of high school students were seen running from a building moments before it went up in flames, a witness has claimed.

A huge fire was seen burning through the seven-storey heritage-listed building on the corner of Elizabeth Street in Surry Hills, near Central Station on Thursday afternoon.

More than 100 firefighters worked to get the blaze under control at the 111-year-old former hat factory that had been left derelict and unoccupied.

Phu Tang, a locksmith, who works next to the building claimed he saw a group of students running from the site as it was engulfed in flames.

The building caught alight in Sydney's CBD, sending thick plumes of smoke billowing into the sky

The building caught alight in Sydney's CBD, sending thick plumes of smoke billowing into the sky

The building caught alight in Sydney’s CBD, sending thick plumes of smoke billowing into the sky

Mr Tang told reporters the group yelled out to their friend who was on the first floor to get out before they all fled.

The building burst into flames about 4pm and had almost entirely toppled over by 6pm, with chunks of debris from the roof and walls falling onto the street below, while thousands of horrified commuters and workers watched on. 

The fire caused a ‘tornado of smoke’ and the heat could be felt even a block away. 

More to come