The United Nations said credible reports indicated at least 10 people were killed in protests in three cities, the first public estimate of any fatalities by an international body since Wednesday's vote.
The government has not released any estimates on casualties or responded to requests for comment. Reuters could not independently verify the figures.
Protesters have taken to the streets since Wednesday, angered by the exclusion of President Samia Suluhu Hassan's two biggest challengers from the race and what they described as widespread repression.
Witnesses have said police fired tear gas and gunshots to break up some demonstrations.
Police have imposed an overnight curfew in the commercial capital Dar es Salaam over the past two nights after government offices and other buildings were set ablaze. Internet access has been disrupted since Wednesday.
Military and police patrolled the streets of Dar es Salaam on Friday, preventing people from moving around without a valid reason. The government extended a work-from-home order to civil servants.
John Kitoka - a spokesperson for the CHADEMA party which was barred from the election for refusing to sign a code of conduct and had its leader arrested for treason in April - said the party had documented about 700 deaths since Wednesday based on accounts from health workers.
He said protests continued on Friday in several cities, although they had waned in some due to the heavy security deployment.
"We are calling for the protests to continue until our demands for electoral reforms are made," he told Reuters.
The unrest presents a test for Hassan, who won praise after taking office in 2021 for easing repression but has more recently faced criticism from opposition parties and activists after a series of arrests and alleged abductions of opponents.
Hassan has denied allegations of widespread rights abuses. She said last year she had ordered an investigation into reports of abductions, but no official findings have been released.
The electoral commission began announcing provisional election results on Thursday, which showed Hassan winning commanding majorities in a number of constituencies.
Her government issued its first comments directly addressing the unrest on Friday in a message from the foreign affairs ministry to diplomatic missions broadcast on state television.
The message said that, "owing to isolated incidents of breaching to law and order, the Government has heightened security and taken several other precautionary measures.
"The security measures in place are temporary but necessary and normalcy will return shortly," it added.
In Geneva, UN human rights spokesperson Seif Magango told reporters there were credible reports of at least 10 people killed in Dar es Salaam, Shinyanga and Morogoro.
He called on security forces "to refrain from using unnecessary or disproportionate force," and urged protesters to be peaceful.
