(AFP) - Tanzania's main opposition presidential candidate Thursday declared he would not recognise the result of an election he said was marred by irregularities such as ballot box stuffing.

Counting was ongoing on the Tanzanian mainland while the semi-autonomous archipelago of Zanzibar -- which elects its own president and lawmakers -- announced the ruling Chama Cha Mapinduzi (CCM) presidential candidate Hussein Ali Mwinyi as victor.

Tanzanian President John Magufuli took an early lead as results trickled in, winning around 85 percent of the 32 constituency results announced by the Tanzania election commission, out of 264.

The CCM has been in power since independence in 1961, but rights groups have decried a slide into autocracy over the past five years marked by the repression of the opposition.

Magufuli's main challenger, the Chadema party candidate Tundu Lissu, declared the results coming in "illegitimate" and urged his supporters to demonstrate peacefully, while asking the international community not to recognise the outcome.

"Whatever happened yesterday was not an election, and thus we do not recognise it. We do not accept the result," Lissu told reporters in Dar es Salaam, saying opposition election monitors had been barred from entering polling stations and faced other interference.

"What is being presented to the world is a complete fraud. It is not an election."

The result of presidential elections cannot be contested in Tanzania, though the parliamentary outcome can be challenged.

Lissu, 52, returned to Tanzania in July after three years abroad recovering from 16 bullet wounds sustained in what he believes was a politically-motivated assassination attempt.

His return reinvigorated an opposition demoralised by years of attacks, arrests and a ban on political rallies, with massive crowds seen throughout his campaign.

However several opposition MPs lost seats in long-held bastions, such as Chadema chairman and lawmaker Freeman Mbowe of Hai in the Kilimanjaro region and ACT-Wazalendo party leader Zitto Kabwe in Kigoma on the shores of Lake Tanganyika.

Of 264 parliamentary seats up for grabs, the CCM has so far won 128, losing only one to the small Civic United Front Party.

A statement from the US embassy said "irregularities and the overwhelming margins of victory raise serious doubts about the credibility of the results" and the government's "commitment to democratic values".

- Trouble in paradise -

In volatile Zanzibar, which has a history of poll violence, cheers and loud music wafted over the capital Stone Town as the elections commission declared Mwinyi the winner with 76 percent of the vote.

There was little suspense as to the outcome, as opposition presidential candidate Seif Sharif Hamad and top ACT-Wazalendo party leaders were in jail as the results were announced.

The opposition said 10 people were killed on Monday and Tuesday, and Hamad decried the election as a "military exercise" overshadowed by violence and cheating.

He has accused the ruling party of trying to steal every vote since multi-party democracy was introduced in 1995, and foreign observers have often agreed.

"People who came in to vote, some of them were given three, four ballot papers. When our party agents tried to ask why, they were thrown out," he said.

"If we don't get our rights through the ballot boxes then we have no choice but to demand this right through the streets."

As Hamad and party leaders left to the protest they called, police fired tear gas at the group before bundling them into a truck, party official Salum Abdalla Bimani told AFP. Hamad was already arrested on Tuesday before being released.

Truckloads of police and soldiers who had been patrolling during the election fanned out across the capital Stone Town, causing streets to empty out.

The feared "zombie" private militia of the ruling party swarmed the city, clad in black and with bandannas covering their faces.

- 'Full authoritarianism' -

The opposition and commentators voiced concern about the fairness of the election well ahead of polling.

"This year it was very different. There was lots of intimidation, violence, kidnapping. Two of our polling agents were kidnapped and very badly beaten," said Tanzanian political analyst Aikande Kwayu, who supports the opposition and was on the ground in Hai.

"We knew it wouldn't be free and fair, but nobody expected this," she told AFP.

"I think we have moved from being a democracy to full authoritarianism."

Tanzania's electoral commission said Wednesday evening they had not received complaints about incidents of ballot stuffing.

Most international media were unable to gain accreditation to cover voting on the mainland, and major social media networks were blocked, accessible only through virtual private networks (VPN).

© Agence France-Presse