Zanzibar is flying its new flag as part of the Indian Ocean islands' 41st revolutionary celebrations.
Zanzibaris believe the flag symbolises their separate identity
It is the first time for four decades that the archipelago has had its own flag since uniting with Tanganyika to form Tanzania in 1964.
Zanzibar's government has stressed the adoption of a flag does not mean that this is a move towards independence.
But the BBC's Ally Saleh says Zanzibaris are pleased it symbolises their separate identity.
Inaugurated by President Amani Abeid Karume on Sunday, the flag is significant to Zanzibari politics, as the archipelago already has its own parliament, executive, legal system and national anthem, our correspondent says.
But the protocol for flying the flag is likely to prove complicated, our correspondent says.
It is not to be raised at international forums such as the United Nations or African Union as Zanzibar is not a sovereign nation, Tanzania's Guardian newspaper reports.
And the new design of blue, black and green horizontal stripes with the Tanzanian flag shown in a corner, has not found favour with everyone.
The opposition Civic United Front (CUF), some of whose members want independence from mainland Tanzania, disputed the inclusion of the Tanzanian union flag.
Under the 1964 act of union - after the revolution - Zanzibar was allowed to remain semi-autonomous and to have its own president while benefiting from economic and political clout of the mainland.
But according to our correspondent, one of the main contentions for islanders over the last 41 years has been the economic disparity between Zanzibar and the mainland.
Meanwhile, Zimbabwe's President Robert Mugabe, attending the island's revolution celebrations, delighted the crowds when he spoke in Kiswahili before continuing in English to say both Zanzibar and Zimbabwe share a common history of anti-colonial struggle.
"My country today faces vilification by the west... Like you, we have refused to yield to any sell out deals," Mr Mugabe told a packed stadium.