Zanzibar held huge rallies Sunday ahead of a presidential election, after the opposition raised fears of fraud in the semi-autonomous archipelago that has a history of contested polls.

Tens of thousands of people dressed in green and yellow gathered at the far larger ruling Chama Cha Mapinduzi (CCM) rally just outside of the capital Stone Town, while the opposition meeting was a sea of bright purple five kilometres (three miles) away.

Police fired teargas at small groups of stone-throwing opposition supporters near the site of the ruling party rally shortly after the simultaneous gatherings ended, an AFP reporter witnessed.

The opposition ACT-Wazalendo (Alliance for Change and Transparency-Patriots) rally took place just hours after campaign manager Nassor Mazrui was abducted and held for several hours before being dumped in a forest.

He told the crowd his car was intercepted and he was seized by six armed men. Police said they were investigating.

- 'Ready to die'-

Sectarian and political tensions in Zanzibar -- with a cosmopolitan population of Arabs, Asians and Africans -- are more marked than on the mainland.

The archipelago, known for its ancient spice trade and paradise beaches, joined with then-Tanganyika to form Tanzania in 1964, and has been ruled by the CCM ever since.

Opposition leader Seif Sharif Hamad, whose party calls for independence from the mainland, is taking his sixth shot at office since multiparty democracy was introduced in 1995.

He alleges that every vote was stolen from him, and many foreign observers have agreed.

"The colonisers (mainland) have oppressed us enough, so take this election very seriously ... we are ready to die for Zanzibar," the 77-year-old Hamad, who leans on a walking stick, told a rapturous crowd.

In January 2001 at least 30 people were killed in clashes between police and opposition supporters after a disputed election.

Polls in 2005 were also marred by clashes.

A political deal allowing for more power sharing led to peaceful elections in 2010, but divisions quickly returned and in 2015, the head of the electoral commission cancelled the vote outright.

In 2016, the opposition boycotted the re-run and the CCM was declared the victor.

Heavily armed riot police and solders patrolled the streets on Sunday, the final day of rallies.

The opposition has condemned plans to have security forces vote a day early on Tuesday as a bid to steal the vote, and has urged supporters to go out and cast ballots on that day as well as on Wednesday.

Hamad's party is backed by Chadema, the leading opposition party on the mainland, in return for ACT-Wazalendo's support of Tundu Lissu, who is running for president of Tanzania in a vote scheduled the same day.

As on the mainland, the opposition has denounced a crackdown on freedom and democracy under union President John Magufuli.

"We are tired of the oppressions that are happening in our country," said art student Ishaka Kassim Hussein, 20.