A nighttime fire ripped through a rundown five-story building in Johannesburg that was occupied by homeless people and squatters, killing at least 73 people early Thursday, emergency services in South Africa's biggest city said. 

Some of the people living in the building in South Africa's biggest city threw themselves out of windows to escape the blaze and might have died because of that, a local government official said. Seven of the victims were children, the youngest a 1-year-old, according to an emergency services spokesperson.

As many as 200 people may have been living in the building, witnesses said.

Emergency crews expected to find more victims as they worked their way through the building, a process slowed by the conditions inside. Dozens of bodies were lined up on a nearby side road, some in body bags, and others covered with silver sheets and blankets.

Another 52 people were injured in the blaze, which broke out at about 1 a.m. in the heart of Johannesburg's central business district, Johannesburg Emergency Services Management spokesman Robert Mulaudzi said. 

Abandoned and broken-down buildings in the area are common and often taken over by people desperately seeking some form of accommodation. City authorities refer to them as “hijacked buildings."

Mulaudzi said the death toll was likely to increase and more bodies were likely trapped inside the building. The fire took three hours to contain, he said, and firefighters had only worked their way through three of the building's five floors by mid-morning.

“This is a tragedy for Johannesburg. Over 20 years in the service, I’ve never come across something like this,” Mulaudzi said.

The building's interior was effectively “an informal settlement” where shacks and other structures had been thrown up and people were crammed into rooms, he said. There were “obstructions” everywhere that would have made it very difficult for residents to escape the deadly blaze and which hindered emergency crews trying to work through the site, according to Mulaudzi.

Search teams found 73 bodies. The chance of anyone being found alive hours after the fire broke out was “very slim,” he said.

City officials said 141 families were affected by the tragedy, although they were not able to immediately say how many people were in the building at the time of the blaze. Many of them were believed to be foreign nationals, officials said.

A witness who didn't give his name told television news channel eNCA that he lived in a building next door and heard people screaming for help and shouting “We're dying in here” when the fire started.

Mgcini Tshwaku, a local government official, said there were indications that people lit fires inside the building to keep warm in the winter cold. Officials are looking into the cause of the blaze.

After the fire was extinguished, smoke still seeped out of windows of the blackened building as daylight broke. Strings of sheets and other material hung out of some of the broken windows. It was not clear if people used those items to try and escape the fire or if they were trying to save their possessions.