Bombs have gone off near four churches in the Iraqi capital and one in the northern city of Mosul, police say.
Many women and children were caught in the blast, said officials
At least 11 people are reported killed and dozens of others injured in what seems to be a new tactic by insurgents.
The first blast occurred outside an Armenian church in Baghdad, and three other churches were hit soon after in what looks like an orchestrated attack.
A blast was reported around the same time in Mosul, where a police station was bombed earlier in the day.
Witnesses said a car bomb detonated outside an Armenian church as an evening service was getting under way.
It blew out stained glass windows, and scattered pieces of hot metal across the street. The wreckage of at least three burned out cars was left in its wake.
"I saw injured women and children and men, the church's glass shattered everywhere. There's glass all over the floor," Juliette Agob, who was inside the church at the time, was quoted by the Associated Press as saying.
Ten minutes later, as the emergency services raced to the scene, a second blast went off outside a Syrian Catholic church about 400 metres from the first church.
An ambulance driver said two people had been killed.
Used to number 1 million
Now estimated at 650,000 - about 3% of population
Main communities: Chaldean Catholics, Assyrian
Other rites include: Armenian, Syrian Orthodox, Syrian Catholic, Anglican
Mainly live in Kirkuk, Irbil, Mosul, Baghdad
Two further explosions followed in the capital. The largest number of casualties was in a church compound in southern Baghdad, where at least eight people were killed.
At around the same time as the Baghdad explosions, a suspected car bomb went off outside a church in the northern city of Mosul. One person was reported killed and several people wounded.
"It's a crime. It's Sunday, we were at mass. There were a lot of women and children," Bishop Raphael Kutami at the Syrian church was quoted by AFP news agency as saying.
"There are so many injured and we don't know how many. We were coming out of the church," when the bomb exploded, said another priest at the same church.
The Vatican has condemned the attacks.
"It is terrible and worrying because it is the first time that Christian churches are being targeted in Iraq," said Vatican deputy spokesman Father Ciro Benedettini.
The BBC's Peter Greste in Baghdad says that until now there has been no significant attacks on Iraqi's Christian minority, although they were becoming increasingly concerned about the possibility of violence.
Many Christians run Iraq's alcohol shops, which have been subjected to recent attacks.
An interior ministry spokesman described them as one of Iraq's most respected groups.
But he also said the attackers may have been trying to antagonise the multinational forces in Iraq, who are from mostly Christian countries.
Earlier on Sunday, at least five people were killed and some 50 injured when a 4X4 vehicle drove at speed into a restricted entrance at a Mosul police station and exploded.