Tanzanian rescue workers are continuing to search for trapped survivors in a collapsed building which killed 13 people.

The four-storey block came down at around 9am local time on Saturday in the east African country's busy Kariakoo market, in the centre of the commercial capital Dar es Salaam.

The presidency said 13 people have been confirmed dead, up from an initial figure of five.

President Samia Suluhu Hassan said in a video at least 84 people had been rescued from the rubble.

Dar es Salaam regional commissioner Albert Chalamila said more people were still trapped in the basement floor of the shattered building, without specifying how many.

"We are communicating… and already we have supplied them with oxygen and water," he said. 

"They are stable and we believe they will be rescued alive and safe."

Fire brigade chief John Masunga said the search and rescue operations had been hampered by the many walls making up the structure of the building.

It is not clear why the commercial building collapsed but witnesses told local media that construction to expand its underground business space had started on Friday.

President Hassan said the immediate focus was on the rescue efforts, and a "thorough" audit on the condition of all buildings in Kariakoo would follow.

"The police should also get all the details about the collapsed building from the owner and know what was exactly happening," she said.

In the immediate aftermath of the collapse, hundreds of first responders used sledgehammers or their bare hands to search the rubble for hours before cranes were brought in.

The incident has renewed criticism over unregulated construction in the Indian Ocean city of more than five million people.

Dar es Salaam has been the scene of a frenetic property boom as one of the world's fastest growing cities, with buildings shooting up quickly often with minimal regard of regulations.

In 2013, a 16-storey building collapsed in the city and killed 34 people.