Security forces killed five opposition supporters in poll-related unrest on the archipelago's Pemba island, the main opposition Civic United Front (CUF) party said on Tuesday.
"Police are going house to house beating our people up," a CUF spokesman told Reuters. "We have confirmed one person killed in Wete and four more in Konde (on Pemba). The situation is very bad. We need urgent international mediation."
Police had no immediate comment on the allegation.
Earlier in the day, thousands of opposition supporters, mostly young men who had engaged the police in running battles for the past two days, were sitting outside the CUF's headquarters in the archipelago's main city.
Police ordered them to return to their homes and opposition officials said police had not entered the building.
Officials released a statement on Monday morning claiming that CUF leader Seif Sharif Hamad had won 50.63% of the presidential vote, edging out incumbent President Amani Abeid Karume who took 49.37% of the vote.
Government, electoral and ruling party officials could not immediately be reached for comment on the opposition claim.
"They stole the results," opposition spokesman Salim Bimani said in a mobile phone text message sent from the party's headquarters.
"They use force to silence voters from demanding their rights."
Reporters watched as police used tear gas and water cannon against opposition supporters in Stone Town, a neighbourhood of the archipelago's main city.
Opposition activists have clashed
with police for the past two days
In the second island of Pemba, police, backed by troops, began beating opposition supporters at their party's headquarters, according to witnesses who declined to be identified for fear of retribution.
Two boats, full of infantry troops, left Tanzania's commercial capital of Dar al-Salaam to reinforce the military presence in this semi-autonomous Indian Ocean archipelago.
In Zanzibar's main island, police later pulled away from the opposition headquarters - located along the narrow alleyways that make up Stone Town - as the number of young opposition supporters swelled there, defiantly singing their party's songs.
At least 100 police officers were deployed outside the neighbourhood. Officers sealed off roads leading to opposition strongholds to prevent supporters from joining the protesters who were burning tyres.
At least 12 people were taken to hospitals for treatment after workers from the International Committee of the Red Cross and Red Crescent called for an end to the confrontations to evacuate the injured.
On Monday, opposition leader Hamad said that more than 80,000 supporters had been denied the right to vote through manipulation of the voter register.
He had called for a meeting with the leaders of the ruling Chama Cha Mapinduzi party and the Zanzibar Electoral Commission on Tuesday morning.
Ruling party supporters are said
to have voted multiple times
Official results showed the ruling Chama Cha Mapinduzi, or Revolutionary Party, winning 16 seats on the main island, and Hamad's party winning all 18 seats on the second island of Pemba, an opposition stronghold.
"The results they are announcing are way off what our reports suggest they should be," one opposition official said, asking not be identified out of fear of possible retribution.
Many of the seats awarded to the ruling party on the main island were from known opposition strongholds, he said.
Free and fair
Before the results were announced, observers from African organisations and the Commonwealth of former British territories had called the election generally free and
fair, though the Commonwealth observers noted problems with the voter's list.
Sunday's balloting, following a campaign marked by violence and recrimination, saw voters choose between the socialists who have ruled for more than 30 years and an opposition party promising privatisation and wholesale economic reform.
There were no reliable exit polls and the electoral authorities said full legislative and presidential results would be announced on Tuesday.
Zanzibar united with Tanganyika
in 1964 after the sultan's ousting
The police action on Tuesday morning was considered to be in preparation for the announcements.
Hamad has called for the announcement of results to be suspended until his party's concerns are addressed. He has promised mass protests if the elections are not free and fair.
The sounds of exploding tear gas canisters echoed down the streets of Stone Town for more than an hour on Tuesday morning, with opposition supporters playing a cat-and-mouse game with police down the narrow 17th-century alleys that make up the old city.
Shops remained closed for a third day because of the violence.
Elections also had been scheduled for Sunday on mainland Tanzania, but were postponed to 18 December because of the death of one of the vice presidential candidates.
Zanzibar united with Tanganyika in 1964 after the violent ouster of the Omani Arab sultan to form the United Republic of Tanzania.
Elections in 1995 and 2000 were marred by violence and fraud. Opposition protests over alleged fraud killed dozens in the wake of the 2000 elections.