Baghdad, Iraq (CNN) -- A series of bomb attacks disrupted voting as the polls opened in Iraq's national elections Thursday, leaving a total of 12 people dead and another 47 wounded.
There had been warnings of insurgent attacks around the vote, which is intended to install a 325-member parliament and select Iraq's next government.
Early voting began Thursday for those who will be unable to cast their ballots in Sunday's election, including thousands of army and security personnel.
In the first of three attacks, five civilians were killed and 22 injured when a bomb went off in the Hurriya neighborhood of northern Baghdad, an interior ministry official said.
The bomb had been placed about 500 meters (546 yards) away from a polling center that was going to be used Sunday, but was not open for Thursday's early voting, the official said. It was not clear whether the polling center was the target, as hundreds of them will be open Sunday, most within 500 meters of each other.
In the al-Mansour district of western Baghdad, at least three people were killed and 25 wounded in a suicide bombing outside a polling center, an interior ministry official said. Most of the casualties were Iraqi army soldiers, the official said.
The suicide bomber struck the forces as they were assembling outside the polling center to vote, the official said.
The bomber was disguised as an Iraqi police officer and stopped before reaching the polling center, according to a U.S. military statement.
A second suicide attacker struck another Iraqi army gathering on its way to vote in the Bab Al-Muadham area of central Baghdad, the ministry said. Four soldiers were killed and 10 others wounded in the attack.
Thousands of army and security personnel are among those taking part in Thursday's voting, which is for those who will unable to cast their ballots Sunday. Others voting Thursday include detainees, hospital staff and patients.
As many as 700,000 security forces are expected to vote, according to the U.N. Assistance Mission to Iraq.
There was heavy security around polling centers Thursday, with Iraqi forces blocking streets around them and not allowing any vehicles in.
At one polling center, voters and personnel entering on foot had to go through a number of body searches. Iraqi Air Force helicopters have been active in the skies all day and snipers can be seen on rooftops near polling centers.
A public holiday began Thursday and will run until Monday because of the elections.
U.S. and Iraqi officials had warned of expected violence ahead of the vote.
On Wednesday, three suicide attacks in Baquba, northeast of the capital, killed at least 33 people and wounded dozens more.
It is Iraq's fifth nationwide vote since 2003, but only the second for a full four-year-term parliament with its 325 seats.
"The conduct and outcome of the election will be the most decisive moment for Iraqis' future since 2003," Ad Melkert, the head of the U.N. mission in Iraq, said Monday.
Authorities on Iraqi state television announced special measures for the days surrounding the vote. Since Monday, there has been a ban on motorcycles and bicycles in Baghdad until further notice, and a two-day ban on any vehicles in cities begins Sunday, they said.
Provincial borders will be sealed, preventing movements between provinces, from Saturday to Monday. And international borders and all airports will be shut from midnight Saturday.
A curfew will be in place in cities from midnight to 5 a.m. on Saturday, Sunday and Monday, officials said. Civilians may not carry weapons on those days, they said.
CNN's Jomana Karadsheh and Yousif Bassil contributed to this report.