Police used tear gas to disperse stone-throwing crowds on the outskirts of Zanzibar's main Stone Town as running battles broke out late on Sunday between supporters of the ruling Chama Cha Mapinduzi (CCM) and the opposition Civic United Front (CUF).
"Fifteen of our people were injured," CUF spokesman Salim Bimani said.
However, police could not confirm the number of the injured.
Political tensions are rising in Tanzania before the 30 October poll, particularly in the semi-autonomous Zanzibar islands where elections in 2000 and 1995 sparked CUF claims of fraud.
At least two people have been killed in poll-related attacks and clashes in the politically unstable islands of Pemba and Unguja that make up Zanzibar since late last year.
"We are not happy with what has happened. We will investigate and take action," George Kizuguto, a regional police commissioner in Zanzibar, said.
"We have enough security forces who are monitoring public rallies. Anyone who is behind all the instability will be charged," he said.
Best known to the outside world for its palm-fringed sandy beaches, Zanzibar will be closely watched next month by foreign donors and others as a test of Tanzania's claim to be a model of stability in Africa.
"We have enough security forces who are monitoring public rallies. Anyone who is behind all the instability will be charged"
George Kizuguto, Zanzibar police
The tourist haven forged a union with mainland Tanganyika in 1964 to create Tanzania while keeping its own parliament and president.
With a population of only one million - a fraction of Tanzania's 35 million population - the former Omani colony remains a backward corner despite income from tourism and spices.
Analysts expect the ruling CCM to win at national level, but say the CUF has a good chance in Zanzibar where independent observers agree it may have been robbed of victory in the past.