Zanzibar - Two small blasts rocked streets of Zanzibar on Saturday but caused no casualties, police said in the latest unrest following the annulment of polls on the semi-autonomous Tanzanian archipelago.
The two devices exploded near a busy roundabout in Zanzibar town, sending people running for cover, the report said.
Zanzibar has experienced sectarian and political tensions in recent years - including several grenade explosions - with the unrest affecting the islands' key tourist industry.
Explosive experts earlier detonated a homemade explosive device in the heart of the historic Stone Town district, a Unesco-listed area popular with tourists.
"People found something unusual... they informed the police," Zanzibar police chief Hamdan Omar Makame said, but pointing out that the device had been safely detonated.
"We want to know the motive behind this and we ask the assistance from the public."
After the device was found, Britain's foreign office issued a travel warning saying: "There are heightened tensions throughout Tanzania particularly in Zanzibar.
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"Violence could escalate quickly. If you're in Zanzibar, avoid being out on the streets and avoid travelling into the centre of Stone Town if possible."
Zanzibar's electoral commission said last Sunday's vote on the islands - where the 500 000 registered voters also cast ballots for Tanzania's national president - must be carried out again, citing "violations of electoral law".
The annulment came after a key candidate, Seif Sharif Hamad of the opposition Civic United Front (CUF), declared himself the winner before the results were officially announced.
Hamad on Friday threatened to call for protests if the situation is not resolved by Monday, saying he would not concede victory to his main rival, incumbent president Ali Mohamed Shien of the long-running Chama Cha Mapinduzi (CCM).
"As from November 2, there will be no government here, Shien will not be the president anymore," Hamad said.
There have also been wider tensions around Zanzibar's union with the mainland, with some opposition parties wanting to break ties and return to the independence it briefly enjoyed in early 1964 before merging with Tanganyika.
The CUF promised to campaign for full autonomy if it wins, while the CCM has vowed to maintain the status quo.
On mainland Tanzania, ruling party candidate John Magufuli of the CCM was on Thursday declared the new president of the country.