Internet users across Tanzania have reported some sites, including WhatsApp and Twitter, are being restricted as millions of people vote in the general election.
There have also been reports of violence and that police arrested an opposition leader in the semi-autonomous archipelago of Zanzibar.
Voters have until 16:00 local time (13:00 GMT) to cast their ballots.
Results are expected to be announced within one week.
President John Magufuli, whose Chama Cha Mapenduzi (CCM) party has governed Tanzania for decades, is seeking a second term in office.
His main challenger is expected to be 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.
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”.
