By Drazen Jorgic and Fumbuka Ng'wanakilala
DAR ES SALAAM (Reuters) - Tanzania's opposition Chadema party said on Monday police had detained 40 of its volunteers after a combined presidential and parliamentary election at the weekend, in an incident marring an otherwise broadly peaceful voting process.
Another opposition party, the Civic United Front, said police fired teargas at a crowd of supporters celebrating what the party said was its victory in Sunday's race on the semi-autonomous island of Zanzibar.
Official results are not expected out until later this week. Partial results on Monday could show the direction of voting following the most hotly contested race after more than five decades of rule by the CCM party.
Tanzania has been one of Africa's most stable democracies, although Zanzibar has often been a hotspot for tensions because of strong local opposition and loud Islamist and separatist voices.
The race for president has pitted the ruling CCM's Works Minister John Magufuli against a coalition of opposition parties led by former prime minister Edward Lowassa, who defected from the CCM just three months ago.
The Chadema party, part of the Ukawa coalition Lowassa heads, said the arrest of 40 of its volunteers was part of a government bid to intimidate the opposition.
"Police also confiscated computers and mobile phones of our volunteers who were tallying results of the presidential election," John Malya, a Chadema lawyer, said.
There was no immediate comment from the government. Police Commissioner Paul Chagonja said some arrests were made due to "violations of electoral procedures" at a Chadema center, where volunteers were tallying the vote. He did not give details.
CCM and the opposition parties posted observers at most polling stations, sending results to their own tallying center to get an idea of voting patterns to prevent abuses.
Opinion polls and analysts have forecast a presidential victory for John Magufuli, the CCM candidate. But many expect the party's parliamentary majority to dwindle after the opposition united behind a single candidate for the first time.
Results started trickling in on Monday morning. Magufuli was ahead after just 65,000 ballots were counted in the nation of 47 million with 22.75 million registered voters.
The electoral commission said the presidential winner would be announced on Thursday.
Lowassa, who quit the CCM after the party spurned him as their candidate, said he would not concede defeat if he did not deem the election free and fair. The opposition has in the past accused the government and CCM of vote rigging. Officials deny this.
Both Magufuli and Lowassa have drawn tens of thousands of people to lively rallies, vowing to curb frequent power outages, halt corruption and deliver economic growth that will deliver more jobs to the poor.
In Zanzibar, the Civic United Front (CUF) said police fired teargas at supporters gathered outside the party's headquarters to celebrate the party's victory. CUF said it had won the vote based on its own count.
"The teargas was part of a campaign of fear ... and intimidation to stop people celebrating," CUF official Ismail Jussa said.
(Editing by Edmund Blair and Hugh Lawson)