Suicide bombers attacked three churches in the second largest Indonesian city of Surabaya, killing at least ten people.
During the attacks, which followed each other within minutes, about 40 others were injured. No group has said so far that it has executed them.
Television pictures showed rubble scattered at the entrance of a church.
Indonesia, the most populous country with a Muslim majority, has seen a resurgence of militant Islamists in recent months.
The bombings took place around 7:30 am local time (00:30 GMT). Officials reportedly thwarted attacks against other churches.
Wawan Purwanto, the Indonesian intelligence service, said that the Islamic State-inspired group Jemaah Ansharut Daulah (JAD) was suspected of standing behind the attacks.
He added that the bombings were probably linked to an incident earlier this month, when five members of the security forces were killed during a 36-hour strike with militant Islamist prisoners in a high-security prison on the outskirts of Jakarta.
Separately, the police said they killed four suspected JAD members in Cianjur, West Java, south of Jakarta.
More than 90% of Indonesians are Muslims, but there is also a significant population of Christians, Hindus and Buddhists.
Christian leaders urged rest. "We need not be afraid to face terrorist threats, we must leave it to the government to deal with it," the head of the Indonesian community of churches said in a statement quoted on Sunday by the Jakarta Post.
Indonesian Foreign Minister Retno Marsudi condemned the attack and expressed its condolences to the victims and their families.
What is militancy in Indonesia?
After the country's worst attack on bomb attacks in 2002 202 people died on the popular holiday island of Bali. Authorities brutally attacked extremist groups.
In recent years, however, several attacks have been demanded by the Islamic State (IS).
Four civilians and four assailants were killed in a series of explosions and shootings in central Jakarta in January 2016; the first attack claimed by the group.
In February of this year, several people were injured in a sword attack in a church in Sleman, Yogyakarta. The police said the attacker had previously tried to join the IS in Syria.