At least 29 people were injured over the weekend when supporters of rival parties clashed in Tanzania's semi-autonomous island of Zanzibar ahead of October elections, police and witnesses said on Monday.

Backers of the ruling Chama Cha Mapinduzi (CCM, or Revolutionary Party) and opposition Civic United Front (CUF) fought with metal bars and stones after leaving separate campaign rallies late on Sunday, they said.

Sixteen CUF supporters were admitted at Al-Rahma hospital and 13 from the CCM were admitted at Mnazi Mmoja National hospital, they said, adding that numerous other people sustained minor injuries.

Zanzibar assistant police Commissioner George Kizuguto confirmed the incident but declined to comment further, pending a probe into the attacks that occurred as both sides prepare for the October 30 general elections.

Police said they had arrested and charged nine people with violence related offences later on Monday.

"Police investigation is going on, but we have arrested nine people and charged them," said the Zanzibar deputy director of criminal investigations, Ramadhani Kinyogo.

According to police and witnesses, the violence erupted when youths from the two sides left the rallies and encountered each other about 10km from Zanzibar's main city, Stone Town.

Peter Magwilla, a witness to the rumpus, said rioters staged a dirty protest, smearing houses with human faeces, while others razed thatched huts in Stone Town.

But CUF spokesperson Salum Bimani said they were first attacked in the presence of riot police.

"Our vehicle was attacked by CCM youths, stoning the people inside. The anti-riot police were at the scene, but did not take any action," Bimani said.

The CUF and CCM are locked in bitter battle for political control of Zanzibar and dozens of people were injured in clashes between their supporters in March and April after the upcoming election was set and during voter registration.

CUF presidential candidate for Zanzibar condemned the violence and renewed his accusations that gangs of ruling party youth dubbed "Janjaweed" were behind the skirmishes.

"We strongly condemn incidents and call upon President [Benjamin] Mkapa to use his legal power to have all the training camps for Janjaweed closed down," he told a press conference in Zanzibar.

"We have several times informed Mkapa about Janjaweed in Zanzibar. If these illegal militias are left, then Zanzibar is in danger of becoming an unstable island," he explained.

But the CCM flatly rejected the claims and condemned the Sunday violence.

"We CCM do not have any illegal wing [that is] being trained to harass people. We really condemn all those who are behind the chaos in Zanzibar and we need the police to act seriously," said the party's spokesperson Vuai Ali Vuai.

Both sides have swapped accusations over the violation of a 2001 truce known as the "Muafaka Accord", which was signed in a bid to prevent a repeat of deadly violence on Zanzibar that killed nearly 40 people after Tanzania's last elections in 2000.

The CUF has twice lost to the CCM in elections, in 1995 and 2000, and has repeatedly complained that Zanzibari authorities, particularly the police and the electoral commission, are favouring the CCM by harassing opposition supporters. -- Sapa-AFP