Sydney fire pictures show the moment the building collapsed

The blaze started inside a former hat factory.  (ABC News)

A massive fire took hold of a seven-storey building in central Sydney shortly after 4pm on Thursday.

It brought Randle Street to a standstill as onlookers watched walls collapse and rubble scatter. 

Here are some pictures from the scene showing how the crisis unfolded. 

Witnesses could feel the heat from the flames.  (AAP: Dean Lewins)

Witness Grace Turner said the ground shook when parts of the burning building started to collapse.

"A huge flume of smoke came up and that whole wall just caved in," she said.

"It was the loudest noise. The ground shook."

The ground shook as the building collapsed.  (AAP: Dean Lewins)

People screamed as bricks and large chunks of walls started crashing to the ground on Randle Street. 

Korin Ellis said he came up from central station in time to see the building collapse.

"All you could feel when you were standing there was heat on your face from the flames. You could hear tyres exploding," he said.

"I saw a couple of people running and crying."

Firemen watched on as debris crashed into the street. (AAP: Dean Lewins)

It took more than 100 firefighters and 20 trucks to contain the blaze before 6pm.

It took about two hours to effectively contain the blaze. (AAP Image: Dean Lewins)

Ash and rubble have been thrown some 200 metres from where the blaze started. 

A witness says ash was falling from the sky.  (ABC News: Jack Fisher)

There were no reports of major injuries.