Internet services have also been shut down, while several people have been injured during protests.
Internet watchdog NetBlocks reported a "nationwide disruption to internet connectivity" in a statement on X.
Earlier, police in Tanzania’s main city, Dar es Salaam, fired tear gas to disperse opposition supporters protesting against the country’s general election.
Several people have been injured in the confrontation after groups of protesters gathered along major highways to demand electoral reforms and free political activity.
Protesters lit fires on roads, vandalised buses, and caused extensive damage to bus terminals and other public infrastructure.
"We are tired… We want an independent electoral commission so that every Tanzanian can choose the leader they want," a protester told the BBC.
Reports say that voter turnout in Dar es Salaam was low when polls opened on Wednesday, with many hesitant to show up amid safety concerns.
A police spokesperson had earlier assured the public there was no threat to their safety, saying "people should come out and vote", according to a message posted on social media.
More than 37 million registered voters are eligible to cast their ballots in the presidential and parliamentary election at over 99,000 polling stations.
Polls opened at 04:00 GMT (07:00 local time) and close at 13:00 GMT (16:00 local time). Results are expected within 72 hours, according to the electoral commission.
The vote comes amid divisions among opposition parties - some boycotting the poll, accusing the process of favouring the ruling party, Chama Cha Mapinduzi (CCM). This is Tanzania’s seventh general election since the return of multiparty democracy in 1992.
President Samia Suluhu Hassan, Africa’s only female head of state, is seeking a new term as 17 candidates vie for the presidency.
The election is expected to be a shoo-in for President Samia Suluhu and her ruling party, as the main opposition leader is in in jail on treason charges, which he denies, and his party has boycotted the vote.
Ahead of the election, rights groups condemned government repression, with Amnesty International citing a "wave of terror" involving enforced disappearances, torture, and extrajudicial killings of opposition figures.
The government rejected the claims, and officials said the election would be free and fair. Samia came into office in 2021 as Tanzania's first female president following the death of President John Magufuli.
She was initially praised for easing political repression under her predecessor, but the political space has since narrowed, with her government accused of targeting critics through arrests and a wave of abductions.
The electoral body is expected to announce results in three days after voting ends.
