DAR ES SALAAM-- The family of Tanzanian billionaire Mohammed Dewji, who was kidnapped on Thursday, has offered a 1bn Tanzania shilling reward for information that leads to his rescue.

Dewji, reportedly Africa's youngest billionaire, was taken by masked gunmen outside a swanky gym in the affluent neighbourhood of Oysterbay.

The kidnappers fired shots in the air before driving away with the billionaire, eyewitnesses said.

Dewji, a fitness enthusiast, had no security guards with him and had driven to the gym on his own, Dar es Salaam regional police commissioner Paul Makonda told reporters.

The police have arrested more than 20 people in connection with the abduction but both the motive for the kidnapping and his whereabouts are still unclear.

Police believe that two white men were behind Dewji's disappearance.

The family hoped that the reward will help the police's investigation, spokesperson Azim Dewji told a packed press conference.

"We want to assure anyone with the information about the whereabouts of our son to come forward and we will treat their information as secret," he said.

Financial magazine Forbes puts Dewji's wealth at $1.5bn, and has described him as Tanzania's only billionaire.

Dewji, locally known as Mo, is credited with turning his family business from a wholesale and retail enterprise into a pan-African conglomerate. His company, MeTL, has interests in textile manufacturing, flour milling, beverages and edible oils in at least six African states.

Dewji served as a ruling party MP for a decade until 2015.