Civil society groups in Tanzania have called for the nullification of Wednesday's general election and the formation of an inclusive transitional arrangement, saying the vote was neither free nor competitive after top opposition contenders were locked out of the race.

Their demand came as police in Dar es Salaam on Thursday fired gunshots and tear gas to disperse crowds that returned to the streets for a second day, defying a night curfew imposed after violent demonstrations that coincided with polling. Protesters rallied in several neighbourhoods - including Mbagala, Gongo la Mboto and Kiluvya - accusing President Samia Suluhu Hassan's government of tightening repression and staging an election whose outcome was predetermined, according to witnesses.

Unrest was first reported on Wednesday in Dar es Salaam and other urban centres as voters reacted angrily to the exclusion of Hassan's two main challengers from the presidential ballot and to what activists described as widening intolerance for government critics. Police had ordered an overnight curfew in the commercial capital, a city of more than seven million people, and by Thursday morning internet access was still heavily disrupted nationwide.

Tanzanian human rights activist Tito Magoti said he had received reports of at least five people killed in Wednesday's protests, while a diplomatic source, who requested anonymity, said there were credible reports of at least 10 deaths in Dar es Salaam alone. Reuters said it could not independently verify the casualty figures, and government and police spokespeople did not respond to requests for comment.

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Even as security forces moved to quell the unrest, state broadcaster TBC - which had made scant mention of the clashes - began airing provisional presidential results showing Hassan with overwhelming margins in multiple constituencies.

The turbulence also affected transport. Britain's Foreign, Commonwealth and Development Office said some international flights to and from Dar es Salaam had been cancelled, and that airports in Arusha and near Mount Kilimanjaro were closed.

Despite the clampdown, protesters using the Zello walkie-talkie app were heard discussing fresh actions, including marches on government offices. "The people are rewriting our political culture from being cows, if I use this word respectfully, to being active citizens," Magoti said, adding that the long-held view of Tanzanians as non-confrontational was being challenged.