Surabaya: (Reuters) Indonesia is the world's largest Muslim-majority country and has seen a recent resurgence in homegrown militancy.
Police told media the attacks were carried out by "suicide bombers" and warned the toll could rise further.
East Java police spokesman Frans Barung Mangera said explosions took place in three churches and 35 people had been taken to hospital.
"There is one location where we can't enter yet," Mangera told reporters near the scene of one of the bombings.
Television footage showed one church engulfed in fire, with thick, black smoke billowing up.
Media reports said a woman with a younger child and a teenager had just entered one church and was being questioned by security when the bomb exploded. Television images showed toppled motorcycles and debris scattered around the entrance of one church and police cordoning off areas as crowds gathered.
Authorities were also investigating whether there was an explosion at a fourth church.
There was no immediate claim of responsibility for the attacks.
The bombings come days after Islamist militant prisoners killed five members of an elite counter-terrorism force during a 36-hour standoff at a high security jail on the outskirts of the capital, Jakarta.
Indonesia has had some major successes tackling militancy inspired by al Qaeda's attacks on the United States in 2001. But there has been a resurgence of Islamist activity in recent years, some of it linked to the rise of Islamic State.
The most serious incident was in January 2016 when four suicide bombers and gunmen attacked a shopping area in central Jakarta.
Churches have also been targeted previously, including near-simultaneous attacks on churches there at Christmas in 2000 that killed about 20 people.
Police ordered the temporary closure of all churches in Surabaya on Sunday, and a large food festival in the city was cancelled
