The election campaign started in Tanzania's semi-autonomous island of Zanzibar on Monday, with the main opposition expressing renewed concerns that the vote may be rigged.
"We shall explore all avenues to see that this year's polls are free, fair, transparent and peaceful," Seif Shariff Hamad, the presidential candidate for the main opposition Civic United Front (CUF) party, told a campaign rally in Zanzibar's capital Stone Town.
Opposition concerns that Zanzibar's electoral commission and the ruling party are plotting to rig the 30 October legislative and presidential increased in August when the commission cancelled a contract with a South African firm, Waymark Infotech, to cross check the registration of voters.
Also in August, the governments in mainland Tanzania and Zanzibar told international donors that they should not try to interfere in the elections.
The islands of Zanzibar and Pemba merged with mainland Tanzania in 1964 while maintaining their own semi-autonomous government, parliament and judiciary.
Hamad said if there was election fraud, "We shall start organising our supporters."
Violence following the 2000 polls left around 40 people dead and many more injured and displaced. In that vote CUF came a close second to the ruling Chama Cha Mapinduzi party.