Tanzanian authorities have arrested six people, including four from Saudi Arabia, in connection with an attack that killed two people and injured 30 others during a mass at a church, officials say.
Jakaya Kikwete, the Tanzanian president, called Sunday's blast, which happened in the northern town of Arusha, an "act of terrorism".
"This is an act of terrorism perpetrated by a cruel person or group who are enemies of the country," Kikwete said in a statement on Monday.
The attack on the church is one of the first such incidents to hit Tanzania.
Officials gave no indication as to who might have carried out the attack, but tensions have been high between Tanzania's Christian and Muslim communities in recent months.
Magesa Mulongo, Arusha's commissioner, confirmed six people had been arrested, two from Tanzania and four from Saudi Arabia.
"Investigations are ongoing," Mulongo said, adding that the four Saudis had arrived at Arusha airport on Saturday.
The two Tanzanians arrested were Christian, he added, but gave no further details.
The blast occurred outside Saint Joseph's Roman Catholic church in Arusha, a town popular with tourists visiting the popular Serengeti National Park and snowcapped Mount Kilimanjaro.
First ever mass
The newly built church, in the Olasti district on the outskirts of Arusha, was celebrating its first ever mass when the blast occurred. People were squeezed into the church building as well as sitting on benches outside.
Archbishop Francisco Montecillo Padilla, the Vatican's ambassador to Tanzania, was attending mass at the church but was not harmed, officials said.
Kikwete, who said he was "shocked and deeply saddened" by the reports of the explosion, called on people to remain calm while police investigated the attacks.
"We are ready to deal with all criminals including terrorists and their agents who are based in the country or externally," he said.
After the attack, worshippers accused the police and the government of failing to properly protect them.
'Community of Islam'
In February, a Catholic priest was shot dead outside his church on the largely Muslim archipelago of Zanzibar, the second such killing in recent months. A church was also set on fire on the island in February.
In March, 52 followers of controversial Muslim cleric Sheikh Ponda Issa Ponda were jailed for a year for violent riots in October in Zanzibar's commercial city of Dar es Salaam, sparked by rumours that a 12-year-old boy at a Christian school had urinated on a copy of the Quran.
Ponda is the head of Jumuiya ya wa Islamu, or the "community of Islam", a group not recognised by the Tanzanian government.
Last month, in the far south of Tanzania, police fired tear gas to disperse about 200 Christian rioters attempting to torch a mosque over an argument over who should be allowed to slaughter animals.
About half of Tanzanians are believed to be Christian, and around a third of the population to be Muslim, although there are no official figures.
In neighbouring Kenya - whose troops invaded southern Somalia in 2011, prompting warnings of revenge by al-Qaeda linked al-Shabab fighters - several churches have been targeted in attacks similar to the Arusha blast.