Security forces used tear gas, water cannons and sticks to disperse taunting opposition supporters in Zanzibar on Monday in a second day of violence over an election on the semi-autonomous Tanzanian islands.
Both main parties claimed to be ahead in Sunday's presidential and parliamentary polls, based on early results.
The opposition Civic United Front (CUF) said that with more than two-thirds of votes counted, it had 54.2 percent versus 45.7 percent for the government, in power for the last four decades both on the islands and on mainland Tanzania.
But the ruling Chama Cha Mapinduzi (CCM or "Party of the Revolution") said it had taken the first seven of 50 legislative constituencies being fought and was on course for victory.
Opposition supporters took to the streets of historic Stone Town, Zanzibar's main population centre, from daybreak, dancing, singing and banging drums to proclaim victory.
But the situation quickly degenerated into a running battle, with police and soldiers firing tear gas into narrow alleys where youths taunted them and threw stones.
Reuters reporters saw about 20 people bundled into vans and taken away, some beaten in full view of foreign media and international election observers out in force on the islands.
The violence tailed off in early afternoon.
"We have freedom today at last after 40 years," said Mohamed Ahmed, 21, holding a poster of CUF leader Seif Sharif Hamad. "We have won. Goodbye CCM!" sang others.
Opposition presidential candidate Hamad, whose party wants privatisation and democratic reforms, said the results his party gave out were based on results sheets seen by his agents.
But he also expressed concern over possible fraud, saying 80,000 of about 500,000 registered voters had not been able to cast ballots, an unusually large number for Zanzibar, where turnout is usually very high.
He did not explain why so many had been unable to vote.
"This opens the way for cooking up results," he said. "They should postpone the announcement of the presidential results until they get all these shortcomings resolved."
Government spokesman Vuai Ali Vuai dismissed opposition claims of victory. "Our friends want to make chaos by telling the world results before they are counted ... We have great hope we will be the winner," he told Reuters.
Zanzibar Electoral Commission Chairman Masauni Yusuf Masauni told reporters there had been no serious problems: "There were no major irregularities. Of course it was free and fair."
The opposition says rigging on the mainly Muslim islands of 1 million people denied it the local presidency at the last two elections. It said it would be further ahead this time were it not for "widespread irregularities" again on Sunday.
The government - which says the international community is biased towards the opposition - blames the CUF for violent clashes on polling day, mainly in Stone Town.
The ruling CCM has held Zanzibar since a 1964 revolution against Arab rule and says it is the guarantor of stability.
It has socialist roots and introduced multi-partyism across Tanzania in 1992. Local President Abeid Amani Karume said on Sunday his party "had the numbers" and would win.
Two past polls in Zanzibar have also been marred by violence - including some 35 deaths in protests after the 2000 vote - sullying Tanzania's reputation for stability.
The east African nation is a favourite of the foreign donor community. Zanzibar is the only place where the government's power is seriously challenged by the opposition CUF.
Foes accuse the opposition of being pro-Arab and having a secret separatist agenda for Zanzibar. But analysts doubt that and the CUF has denied it. The opposition says it will invite CCM into power on Zanzibar if it wins.
Foreign observers said the violence was disappointing, but cautioned it may have been localised in Stone Town. There were reports of irregularities, but not of more violence, across the rest of the main island Unguja and the second island Pemba.
The African Union gave the poll a guarded endorsement. "In spite of incidents and accounts of irregularities we think that largely the election went well," AU observer Baleke Mbete said.