Tanzania's main opposition party said on Tuesday it did not recognise results announced so far from a weekend presidential and parliamentary election due to "widespread rigging", after a broadly peaceful vote that the ruling party said it won.

Tanzania has been one of Africa's most politically stable nations, ruled for half a century by the Chama Cha Mapinduzi (CCM) party despite the CCM president being changed often. But Sunday's vote was the most hotly contested in CCM's history.

The opposition has often complained about abuses in past votes, but this challenge carries more weight because Chadema and other major opposition parties have united in a coalition for the first time, fielding a single presidential candidate.

Full and final results are not expected until Thursday.

"The ongoing presidential results being announced by the National Electoral Commission are deeply flawed," Chadema Chairman Freedom Mbowe said, noting that the party did not recognise the results being released.

"There is widespread rigging. This election is a shambles," he told a news conference with leaders from other members of the Ukawa opposition coalition, which is fielding former Prime Minister Edward Lowassa as its presidential candidate.

Lowassa did not attend.

Mbowe said 166 opposition supporters had been arrested, including IT experts who were carrying out the party's own tally of results. The party previously said 40 had been detained.

Parties in Tanzania send representatives to polling stations and then put together their own tally of results.

"These arrests were aimed at destroying our capacity to independently verify the results," Mbowe said.

The chairman of the National Electoral Commission, Damian Lubuva, earlier dismissed allegations of any voting abuses. "There is no favouritism whatsoever," he said. "The results that we are announcing reflect the will of the people."

The ruling CCM party, which has said it was on track to win the presidency and retain its big majority in parliament based on initial results, also said the election was fair.

"Conditions for fair and transparent elections existed," CCM campaign director January Makamba told Reuters.

He noted there had been some challenges in conducting the vote, leading to some constituencies voting a day late, but he said that did not change "the direction" of the results.

The CCM's presidential candidate, John Magufuli, is pitted against Lowassa, a popular figure although he only quit CCM in July after the ruling party snubbed him as its candidate.

The semi-autonomous island of Zanzibar, traditionally a hotspot during elections, has also witnessed tensions again.

The island's opposition Civic United Front declared victory in the vote on Monday while counting went on. Police fired tear gas to disperse supporters when they gathered to celebrate.

Zanzibar has a strong opposition and loud Islamist and separatist voices, although the ruling party has also said it expects to win overall on the island.

Report by Fumbuka Ng'wanakilala