Protests broke out in Dar es Salaam and several other cities during the vote on Wednesday, with demonstrators infuriated by the exclusion of President Samia Suluhu Hassan's two biggest challengers from the presidential race, as well as what they say is increasing repression of government critics.
Police ordered an overnight curfew in Dar es Salaam, a city of more than seven million people, and internet access remained disrupted across the country.
Tito Magoti, a Tanzanian human rights activist, told Reuters he had received reports of at least five deaths in Wednesday's protests.
A diplomatic source said there were solid reports that at least 10 people had been killed in Dar es Salaam alone.
Reuters could not independently verify reports of casualties and spokespeople for the government and police did not respond to requests for comment.
The state-run Tanzania Broadcasting Corporation, which has hardly mentioned the unrest, began airing the announcement of provisional presidential election results, which showed Hassan winning commanding majorities in various constituencies.
Dozens of protesters returned to the streets of the Mbagala, Gongo la Mboto and Kiluvya neighbourhoods on Thursday, where police fired gunshots and tear gas, the Reuters witness said.
On the Zello app, which allows a smartphone to function like a walkie-talkie, some protesters discussed plans for further demonstrations, including marches on government buildings.
The unrest presents a major test for Hassan, who won plaudits after taking office in 2021 for easing repression of political opponents and censorship that had increased under her predecessor John Magufuli.
In recent years, however, rights campaigners and opposition candidates have accused the government of unexplained abductions of its critics.
In a post on his Instagram account, government spokesperson Gerson Msigwa said all civil servants should work from home on Thursday except for those whose duties require them to be present at their workplaces.
Tanzania's main opposition party CHADEMA had called for protests during the election, which it said amounted to a "coronation" of Hassan.
CHADEMA was disqualified in April from the election, which also included votes for members of parliament and officials for the semi-autonomous Zanzibar archipelago, after it refused to sign a code of conduct, and its leader Tundu Lissu was charged with treason.
The commission also disqualified the candidate for opposition party ACT-Wazalendo, leaving only minor parties to take on Hassan.