He was abducted by two foreigners, the Swahili Times reported on Twitter, citing Mambosasa.
The motive for the kidnapping is still unknown, although police expect this to be a "ransom" kidnapping.
News of the incident spread fast as social media users shared the development, sending shockwaves the country.
His conglomerate, the METL Group, operates in over 35 industries as diverse as trading, agriculture, manufacturing, energy and petroleum, financial services, mobile telephony, infrastructure and real estate, transport, logistics and distribution.
Mr Dewji studied at Georgetown University in Washington DC.
Mr Dewji is also the main shareholder in Simba football club, based in Dar es Salaam.
Makonda had said that Dewji had no security detail with him and drove to the gym on his own.
Mr Dewji served as a ruling party MP for a decade until 2015.
"Police are looking for suspects and have already made arrests", Mr Lazaro said.
The kidnappers fired shots in the air before driving away with the billionaire, eyewitnesses said. Another vehicle remained outside.
They then pulled him into their auto and sped off while shooting in the air.
"All our borders in and out of the Dar es Salaam region are being monitored to make sure the cars involved are stopped or confiscated and the culprits are brought to justice as soon as possible", Regional Commissioner Paul Makonda said at a news conference.
Three women are among the 55 richest people in Africa, according to Ventures magazine.