Protesters say the East African island should pull out of mainland Tanzania and adopt political Islam.
There are an estimated 9,000 heroin addicts on the small East African island of Zanzibar – one of the highest rates of usage in the world.
Zanzibar’s container port is along several key shipping routes that ferry the drug from Asia to big markets in the US and Europe.
And residents say it is relatively easy to smuggle heroin on and off the island.
Although the drug is illegal in Zanzibar, it is almost impossible for the authorities to know what goods are coming or going.
Al Jazeera’s Malcolm Webb reports from Stonetown.