A polling station clerk holds up a ballot paper during vote counting on October 25, 2015, on the semi-autonomous archipelago of Zanzibar
Dar es Salaam (AFP) - Votes were being counted Monday in what is expected to be Tanzania's tightest election race ever, with the governing party facing the first major challenge to its dominance in decades.
But the opposition Chadema party has alleged rigging in Sunday's presidential, general and local elections.
"There are allegations of electoral fraud," Chadema spokesman Tumaini Makene told reporters late Sunday.
John Magufuli of the long-ruling Chama Cha Mapinduzi (CCM) is seen as the narrow favourite to beat ex-prime minister Edward Lowassa, a CCM stalwart who recently defected to Chadema, which is heading a coalition of opposition parties.
On Zanzibar, the semi-autonomous archipelago which also voted for its own leader, the main opposition presidential candidate declared himself the winner, ahead of any official announcement of results.
Analysts have warned that the unusually tight race could spark tensions, with the opposition providing the first credible challenge to the CCM since the introduction of multi-party democracy in 1995.
Voting was delayed in some districts, including in some suburbs of the main city Dar es Salaam, after ballot papers were delayed, top National Elections Commission (NEC) official Kailima Ramadhani said.
- Opposition claim mass arrests -
In the southwestern Sumbawanga region, "armed people attacked a vehicle hired by NEC and burned ballot papers," Ramadhani said, adding that new voting dates for those districts would be organised.
While voting otherwise passed off peacefully, senior Chadema official Mwesiga Baregu told reporters they were concerned at reports of "a number of reported interceptions of stuffed (ballot) boxes."
The opposition claims could not be independently verified, but Chadema said it was concerned, adding that police late Sunday arrested some 30 members tabulating results at their tallying centre.
"The goal is to disable us from collecting results and tally them," Lowassa said.
Election officials said they expect the results of the presidential race within three days, but some early results should begin arriving on Monday.
"We seem to be heading to results that may not be credible, and therefore might not be accepted not just by political parties, but will not be accepted by the voters themselves," Baregu said.
British High Commissioner Dianna Melrose said she was generally impressed with the polls, but said there was concern at "some cases where voting materials were delayed."
CCM election co-ordinator Yusuf Makamba criticised what he called the opposition's "inflammatory statements", warning that comments suggesting they might not accept official results "may spark unrest".
Many believe 55-year old Magufuli -- currently minister of works, for which he earned the nickname "The Bulldozer" -- will face a tough challenge from Lowassa, 62.
Lowassa was prime minister from 2005 until his resignation in 2008 over corruption allegations that he denies, and has for years been a CCM loyalist, but on the campaign trail he called for an end to the party's rule.
- Zanzibar tensions -
Outgoing President Jakaya Kikwete, who is not running having served his constitutional two-term limit, ordered the police to boost security to ensure calm in the country of 52 million people, of whom 22 million were registered to vote.
On Zanzibar, a popular tourist destination, police fired tear gas to disperse crowds on polling day, and Britain's Foreign Office on Monday warned there "may be heightened tension and unrest" after the vote.
Leading candidates in the Zanzibar vote are incumbent President Ali Mohamed Shein of the ruling CCM, and current Vice-President Seif Sharif Hamad from the opposition Civic United Front (CUF), who shared power in a unity government.
Hamad, 71, claimed on Monday to have seen official documents of final voting figures, although there has been no announcement from the electoral commission and the figures could not be verified.
"There is no question... the ruling party has been defeated for the first time since 1977," Hamad told reporters.
"I have won by 52.87 percent, against the incumbent president Dr Ali Mohamed Shein's 47.13 percent," he said.
The declaration is likely to raise tensions on the islands. Zanzibar has experienced sectarian and political tensions in recent years, including several grenade explosions -- with the unrest affecting the islands' key tourist industry.