Eight people - including four of Saudi origin - have been arrested over an explosion that killed two people at a church in the northern Tanzanian city of Arusha on Sunday, police say.
President Jakaya Kikwete condemned the blast as an "act of terrorism".
No group has said it was behind the explosion, which left at least 50 people wounded.
Militant attacks are relatively rare in Tanzania, unlike neighbouring Kenya and Somalia.
Al-Qaeda-linked suicide bombers killed more than 200 people in simultaneous attacks in Tanzania and Kenya in 1998.
Tanzania has seen a rise in sectarian violence between Christians and Muslims in the past year.
There are no official records of religious affiliation in Tanzania so it is not clear whether Muslims or Christians form the majority.
Regional commissioner Magesa Mulongo told the BBC that eight people - four of Saudi origin and four of Tanzanian origin - had been arrested, as investigations continue into the blast.Two of the Tanzanians were Christians, while the four Saudis had arrived at Arusha airport a day before the blast, AFP news agency quotes him as saying.
The explosion occurred at the official opening of the new Roman Catholic church in the suburb of Olasiti in Arusha, which is mainly Christian.
The Vatican's ambassador to Tanzania and the archbishop of Arusha were attending, but were not hurt.
Eyewitnesses said a bomb had been thrown from a motorcycle, AP news agency reports.
The attack happened despite heavy security presence in the area.
Mr Kikwete said he was "shocked and deeply saddened" by the blast.
"We are ready to deal with all criminals, including terrorists and their agents, who are based in the country or externally," he added.
Last month, police in southern Tanzania used teargas to disperse about 200 Christian rioters attempting to set fire to a mosque following a dispute over animal slaughtering.
In February, a Catholic priest was shot in the head on the largely Muslim island of Zanzibar.
Last year Muslim cleric Sheikh Ponda Issa Ponda was arrested over attacks on churches, following rumours that a Christian boy had urinated on a copy of the Koran.