The death toll rose above 100 on Friday after a ferry capsized and sank on Lake Victoria, Tanzania state radio reported.
The toll is likely to rise further as search and rescue operations continue after Thursday afternoon's disaster, John Mongella, commissioner for the Mwanza region, told The Associated Press.
President John Magufuli urged the country to remain calm.
At least 37 people were rescued from the sunken ferry as of Thursday evening, when rescue teams called off their mission overnight.
It is not clear how many people were on board. Such ferries often carry hundreds of people and are overcrowded.
No foreigners have been found among the dead, Mwanza police commander Jonathan Shanna said Friday.
The passenger ferry MV Nyerere was traveling between Ukara and Bugolora when it sank, according to the government agency in charge of servicing the vessels.
Accidents are often reported on the large freshwater lake surrounded by Tanzania, Kenya and Uganda.
Some of the deadliest have occurred in Tanzania, where passenger boats are often said to be old and in poor condition.
In 1996, more than 800 people were killed when the passenger and cargo ferry MV Bukoba sank on Lake Victoria.
Nearly 200 people died in 2011 when the MV Spice Islander I sank off Tanzania's Indian Ocean coast near Zanzibar.