Arusha - Tanzanian police arrested six people on Monday including Saudi nationals over a deadly bombing at a church mass that President Jakaya Kikwete described as an "act of terrorism".
Two people were killed and 64 wounded in the attack on the church in the northern town of Arusha on Sunday, officials said, one of the first such incidents in Tanzania.
"This is an act of terrorism perpetrated by a cruel person or group who are enemies of the country," Kikwete said in a statement.
Four of those arrested are from Saudi Arabia and two from Tanzania, with police hunting for more suspects, officials said.
Officials have given 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.
"Investigations are ongoing," Arusha's commissioner Magesa Mulongo said, adding that the four Saudis arrested had arrived at Arusha airport on Saturday.
The two Tanzanians arrested were Christian, he added, but gave no further details.
Internal Affairs Minister Emmanuel Nchimbi told parliament that police and army explosives experts were examining the blast site.
"Preliminary investigations show that the bomb was thrown into the church compound, but we don't know its nature," Nchimbi said, adding that there were "indications that more people are involved".
"We must protect national unity, peace and tranquility of our country at any costs," he added.
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.
The newly built church, in the Olasti district on the outskirts of Arusha, was celebrating its first ever mass at the time of the attack, and people were squeezed into the church building as well as sitting on benches outside.
The Vatican's ambassador to Tanzania, Archbishop Francisco Montecillo Padilla, was attending mass at the church but was not harmed, officials said.
Kikwete, who said he was "shocked and deeply saddened" by the incident, 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," said Kikwete, who cut short an official visit to Kuwait following the attack.
Worshippers accused the police and the government of failing to properly protect them.
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 Zanzibar in February.
Jumuiya ya wa Islamu
In March, 52 followers of controversial Muslim cleric Sheikh Ponda Issa Ponda were jailed for a year for riots in October in the commercial capital Dar es Salaam, sparked by rumours that a 12-year-old boy at a Christian school had urinated on a copy of the Qur'an.
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 around 200 Christian rioters attempting to torch a mosque over an argument over who should be allowed to slaughter animals.
Around 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 the Al-Qaeda linked Shabaab insurgents - several churches have been targeted in attacks similar to the Arusha blast.
While Tanzania does not have troops in Somalia, it is home to Islamist groups connected to radical groups in the wider region including the Shabaab, according to United Nations experts.