In an apparently coordinated attack timed to coincide with evening prayers, four blasts hit churches in Baghdad and two in the northern city of Mosul on Sunday.
At least 12 worshippers died at a Chaldean church in southern Baghdad when an attacker detonated himself and his vehicle in the building's car park.
An explosion at the Armenian church in Baghdad shattered stained glass windows and sent chunks of hot metal flying. Another bomb exploded 15 minutes later at a nearby Assyrian church.
"Worshippers were inside the church and during the service a bomb went off," said worshipper Shakib Musa Jibril.
An ambulance driver said two people were killed in the explosion at the Assyrian church and several wounded.
"We are expecting a huge number of casualties," an Interior Ministry source told journalists, confirming five explosions.
"Those are terrorist acts against the Iraqi people and against Iraq, and we're going to finish them [the perpetrators]," Interior Minister Falah al-Naqib told reporters at the Assyrian church.
The Vatican condemned the blasts - the first attacks on churches during the 15-month insurgency - echoing concerns among Iraqis that they aimed to inflame religious tensions.
"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.
US Colonel Mike Murray of the 1st Cavalry Division said at least 50 people had been wounded at one church alone, some seriously.
In Mosul, officials said at least one person was killed in a blast at a church and 15 wounded.
The US military said the attackers fired a rocket at the Mar Polis Catholic Church before detonating a car bomb. It put the toll from the attack at one dead and seven wounded.
There are about 700,000 Christians in Iraq, most of them in Baghdad.
Several recent attacks have targeted alcohol sellers throughout Iraq, most of whom are Christians of either the Assyrian, Chaldean or Armenian denominations.