Polls have opened in Tanzania's general election, following nationwide reports of restricted internet access plus the arrest of an opposition leader in the semi-autonomous islands of Zanzibar.
Voters have until 16:00 local time (13:00 GMT) to cast their ballot.
Results are expected to be announced within one week.
President John Magufuli, whose Chama Cha Mapenduzi (CCM) party has ruled Tanzania since independence in 1961, is seeking a second term in office.
His main challenger is Tundu Lissu of the biggest opposition party, Chadema, who survived an assassination attempt three years ago. He returned from Belgium in July where he had undergone rounds of treatment for gunshot wounds.
In all, 15 candidates are running for president - including former foreign minister Bernard Membe, an ex-colleague-turned-critic of Mr Magufuli who defected from the ruling party and now leads ACT Wazalendo.
The BBC's Athuman Mtulya in Dar es Salaam says both Mr Magufuli and Mr Lissu have drawn huge crowds during their campaigns.
However Mr Lissu's campaign was suspended for seven days after being accused of sedition by the electoral commission. He had reportedly said that Mr Magufuli was planning to rig the elections, but the commission said there were no such attempts.
Under Tanzania's electoral rules, the winning candidate requires a simple majority to become president.
Some 29.1 million people are registered to vote in Wednesday's general election - six million more than than the previous polls in 2015.
They include residents of Zanzibar who vote for the islands' leaders - including the election of a Zanzibari president - as well as in Tanzania's national election.
Tensions in Zanzibar
Unlike in mainland Tanzania, Zanzibar's main opposition party is ACT Wazalendo. Their leader was arrested at a polling station on Tuesday morning and later released according to multiple reports.
But police have not commented on Maalim Seif Sharif's reported arrest, and have also denied reports that officers shot three people dead on the island of Pemba on Monday.
Zanzibar has a history of contested polls, including in 2015 when they were annulled for not being free and fair. The opposition boycotted the re-run and the ruling CCM party's candidate was declared the winner.
Twitter warned on Tuesday it was "seeing some blocking and throttling" of its services in Tanzania ahead of Wednesday's polls, and appealed for the respect of "basic human rights".
Internet users across the country have also reported difficulties when trying to download photos or videos on the hugely popular messaging platform WhatsApp.
Tanzanian authorities have not addressed the apparent internet restrictions, and the country's regulatory body has not responded to the BBC's request for comment.