Sites, including WhatsApp and Twitter, are being restricted in Tanzania 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.
President John Magufuli, whose 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.
Results for the presidential and parliamentary elections are expected to be announced within one week.
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”.
— Twitter Public Policy (@Policy) October 27, 2020
Internet users across the country have also reported difficulties when trying to download photos or videos on the hugely popular messaging platform WhatsApp, according to internet block observers Net Blocks and ProtonVPN.
Confirmed: Widespread disruption to social media registered across #Tanzania on eve of elections; high impact to Twitter, WhatsApp, Instagram and Google services on Vodacom, Airtel, Tigo, Halotel and Zantel; incident ongoing 📉 #TanzaniaElections2020
— NetBlocks.org (@netblocks) October 27, 2020
To get round the internet block, people can use a virtual private network (VPN) but one such VPN has said that people’s efforts to sign up are being thwarted as text message verification is also being blocked.
Internet is being blocked in #Tanzania since yesterday, the eve of an important election. Authorities are now trying to block VPN services. ProtonVPN signups are impacted because SMS verification code messages are being blocked (email verification works, for now). https://t.co/nXG6ygRYVR
— ProtonVPN (@ProtonVPN) October 28, 2020
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Tanzanian authorities have not addressed the apparent internet restrictions, and the country’s regulatory body has not responded to any request for comment.