The family of Tanzania's only billionaire Mohammed Dewji, who was kidnapped last week in Dar es Salaam, has announced a reward of nearly half a million dollars for information on his whereabouts.

Gunmen snatched Dewji, 43, listed by Forbes as Africa's youngest billionaire, as he entered a hotel gym in Tanzania's economic capital last Thursday morning.

"The family is offering a reward of a billion (Tanzanian) shillings ($437,000) to any person who has information leading to the whereabouts of our son," family spokesman Azim Dewji told a press conference Monday.

Interior Minister Kangi Lugola said Saturday that security forces were "working day and night" to find Dewji, with 20 people having been arrested so far.

Officials have implicated foreigners in the abduction, saying he was taken by "whites".

Dewji is chief executive of the MeTL Group, which operates in a dozen countries and has interests in agriculture, insurance, transport, logistics and the food industry.

According to Forbes, he is worth $1.5 billion and ranks 17th on the list of African billionaires.

He was a member of parliament from 2005 to 2015, and in 2013 he became the first Tanzanian to feature on the cover of Forbes magazine.

Two years later, he was named Forbes' Africa Person of the Year.

Dewji is also the main shareholder in Tanzania's Simba FC football club.