Tanzania's President John Magufuli is leading the presidential election count, preliminary results showed on Friday, after a contest this week dismissed by the opposition as a "travesty" of a poll due to widespread irregularities.

Magufuli's CCM party, a version of which has held power in Tanzania since independence from Britain in 1961, has already retained power in the semi-autonomous archipelago of Zanzibar with 76% of the vote.

Dozens of opposition party officials and members were arrested in Zanzibar on Thursday and at least one is in hospital with severe injuries after he was beaten by the police, who have not commented on the incident.

Magufuli has garnered 84.67% of the vote from 142 constituencies that have reported so far, just over half of the total, the state broadcaster TBC said, citing the electoral commission.

Tundu Lissu, his main challenger, has said that he won't accept the eventual election results - expected within the next few days - due to the irregularities. Lissu is running for the Chadema party.

The US Embassy in the East African country said there had been "credible allegations of significant election-related fraud and intimidation" in Wednesday's poll in which voters were electing a president and lawmakers.

The vote was marred by allegations of arrests of candidates and protesters, restrictions on agents of political parties to access polling stations, multiple voting, pre-ticking of ballots and widespread blocking of social media, the US Embassy said.

Officials at the electoral commission were not immediately available for comment on allegations of irregularities. On Wednesday, the commission denied allegations of fake ballots, saying they were unofficial and unsubstantiated.

Magufuli is seeking a second, five-year term, and has promised voters that he will boost the economy by completing ambitious infrastructure projects he started in his first term.

Zitto Kabwe, the leader of the main opposition party in Zanzibar, ACT-Wazalendo, and Chadema's leader in parliament, Freeman Mbowe, are among dozens of opposition candidates who have lost their seats to the ruling party.