Tanzania's leading opposition presidential candidate on Thursday asked countries around the world not to recognise the eventual results of an election held the previous day, calling it a "travesty" due to widespread irregularities.

Tundu Lissu, the main challenger to incumbent President John Magufuli, told Reuters the defects in the process meant that the results - expected within a week -- could not be trusted.

Magufuli is seeking a second, five year term in presidential and parliamentary polls marked by complaints of restricted internet access and accusations of fraud.

"The results should not be recognised by any country in the world, should not be recognised by the African Union and the Commonwealth," Lissu told Reuters, urging the world to take action against "those who perpetrated this travesty."

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

Lissu, 52, was shot 16 times in 2017 in what remains an unsolved case. He returned from three years in exile in July.

Lissu said his party's polling agents had been prevented into getting into polling stations and that stuffed ballots had been forced into polling stations across the country.

"This was a mockery of elections, a mockery of democracy," he said. He later told reporters "We are not accepting anything that has been done and any results."

Magufuli, 61, is looking to extend the rule of the CCM party, a version of which has held power since independence from Britain in 1961. It won the presidency with 58% of votes in 2015 and now holds about three-quarters of parliamentary seats.

In the race for 264 parliamentary seats, the leader of the opposition in the assembly, Freeman Mbowe, lost his seat in the north to his CCM challenger Saasisha Mafuwe, state broadcaster TBC reported.

While campaigning, Magufuli pledged to boost growth in East Africa's third largest economy with infrastructure projects kickstarted in his first term.

But the opposition and rights groups have complained that his administration has cracked down on critical voices, closing down media outlets and preventing opposition public rallies.

Opposition parties said electoral authorities disqualified dozens of their parliamentary candidates. The government has denied suppressing dissent and the National Electoral Commission has rejected charges of unfair treatment.