Tanzania's main opposition leader says there has been "shameless" widespread vote rigging in Wednesday's elections.
But the National Electoral Commission says the accusations are unfounded.
Internet users have also reported WhatsApp and Twitter, among other sites, have been blocked. Voting has been calm and peaceful on the mainland after reports of violence in Zanzibar on Tuesday.
Polls have now closed and election results are expected within a 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, 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.
What are the allegations?
Opposition parties ACT Wazalendo and Chadema have made accusations of vote rigging.
In a series of tweets, Mr Lissu, from Chadema, said there had been "shameless" election fraud.
He retweeted a video of what appeared to citizens intercepting a bag full of pre-filled ballots, all voting for the CCM.
He said he had received reports indicating widespread irregularities, including opposition polling agents being prevented from accessing polling stations.
The National Electoral Commission chairperson Judge Semistocles Kaijage denied the claims, saying they were unfounded.
What's happening with the internet?
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, according to internet block observers Net Blocks.
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.
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.
Who is John Magufuli?
President Magufuli has styled himself as a stout African nationalist and a devout Catholic waging war against foreign powers seeking to exploit Tanzania.
He has been praised for pushing through big infrastructure projects while critics have said he has chipped away at freedom of expression.
He did not impose a lockdown to restrict the spread of coronavirus and later said that God had spared Tanzania the virus.
However, the government has also stopped publishing figures for the number of people infected with the virus.
Who is Tundu Lissu?
Mr Lissu is expected to be the strongest opposition challenger, as the leader of the biggest opposition party - Chadema.
The 52-year-old lawyer gained a reputation as an outspoken critic of Mr Magufuli.
He survived an assassination attempt three years ago and travelled to Belgium where he underwent rounds of treatment for gunshot wounds.
Flying back into the country in July, he was greeted at the airport by his backers as a returning hero.
The BBC's Athuman Mtulya in Dar es Salaam says both Mr Magufuli and Mr Lissu have drawn huge crowds during their campaigns.
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 - six million more than than the previous polls in 2015.
They include residents of the semi-autonomous archipelago of Zanzibar who vote for the islands' leaders - including the election of a Zanzibari president - as well as in Tanzania's national election.
What happened in Zanzibar?
Unlike in mainland Tanzania, Zanzibar's main opposition party is ACT Wazalendo. Its presidential candidate 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.
This article was published by BBC News.