Christian Solidarity Worldwide is calling for "decisive action" after the deadly bombing of a cathedral in Arusha, Tanzania, on Sunday.
The explosion happened during an inaugural Mass at the cathedral, killing three people and injuring more than 60.
The Vatican's ambassador to Tanzania, Archbishop Francisco Montecillo Padilla, and the Archbishop of Arusha, Josaphat Louis Lebulu, were at the Mass but escaped unharmed.
President Kikwete has described the attack as "an act of terrorism" and vowed to catch the perpetrators.
The bomb was thrown into the cathedral from a motorcycle. Nine people, including three nationals from the United Arab Emirates, one Saudi and five Tanzanians, have been arrested over the bombing.
Mervyn Thomas, chief executive at Christian Solidarity Worldwide, condemned the "senseless" attack.
"Although those arrested have yet to be charged, there is undoubtedly an international element to this attack, which if left unchecked, will severely undermine the stability of the nation," he said.
There has been a string of attacks on Christians in Tanzania this year. In the semi-autonomous region of Zanzibar, a Protestant pastor and Catholic priest were murdered. This was followed by threatening text messages to church leaders from a group calling itself "Muslim Renewal", which claimed responsibility for the killings.
In Geita Region, a Pentecostal pastor was beheaded in the town of Buseresere in March by Muslims angry over the opening of a non-Muslim butcher.
In Mara province, an evangelist was attacked in March by unknown assailants.
Mr Thomas said: "Signs of escalating attacks on Christians in Tanzania are deeply worrying.
"CSW calls upon the Tanzanian authorities to take decisive action to tackle rising extremism and to ensure that justice is done innocent civilians and worshippers are protected from violence and an atmosphere of impunity is not allowed to take hold."