Update - 13.09pm: The death toll has risen above 100 after a ferry capsized and sank on Lake Victoria, Tanzania state radio reported.
President John Magufuli urged the country to remain calm.
It is not clear how many people were on board.
No foreigners have been found among the dead, Mwanza police commander Jonathan Shanna said today.
The death toll has risen to 86 after a ferry sank on Lake Victoria in Tanzania, officials said.
John Mongella, commissioner for the Mwanza region, said the toll is likely to rise as search and rescue operations resume.
The government said the passenger ferry MV Nyerere was travelling between Ukara and Bugolora on Thursday when it sank.
Such ferries often carry hundreds of people and are overcrowded. 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 old and in poor condition.
At least 44 people died when after a passenger ferry capsized on Lake Victoria, a Tanzanian official has said.
The death toll was expected to rise when rescue efforts resumed at daybreak.
Thirty-seven people were rescued after the sinking on Thursday afternoon, Mwanza regional commissioner John Mongella said.
"I cannot speculate" how many people had been on board, he said. "Right now our focus is on rescue."
Such ferries often carry hundreds of people and are overcrowded.
The Tanzania Electrical, Mechanical and Electronics Services Agency, in charge of servicing the vessels, urged patience in a statement as rescue efforts began.
The ferry was travelling between Ukara and Bugolora and capsized near the area of Mwanza, the agency said.
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.