Four people have been arrested by police after an explosion at a ruling party office in Zanzibar.
Attacks took place as voters lined up to register
Tension between the ruling CCM and the opposition CUF has risen in the past month, with several reports of clashes between rival supporters.
General elections are due in October on the archipelago, which enjoys a large degree of autonomy from the central Tanzanian government.
The dead body of a missing CCM official was found in a shallow grave on Sunday.
Police have banned political rallies and voter registration has been suspended twice in an attempt to calm tensions.
No casualties were reported in the blast.
"So far, we have arrested four people in connection with a bomb blast - they are the main suspects as of now," police investigator Ramadhani Kinyogo told AFP news agency by phone from Mpandae where the blast took place.
"We do not know what kind of bomb it was, but our experts have gone to the scene."
The CUF accuses the ruling CCM party of rigging previous elections and using violence and intimidation against its supporters. Some 40 people were killed in violence during the 2000 election campaign.
The CUF enjoys most of its support on the semi-autonomous archipelago but the CCM party has won recent elections on the islands as well as on the mainland.