Iraqi police also said insurgents kidnapped a Japanese engineer, but on Thursday officials in Tokyo cast doubt on the report, saying they had no information on the incident.
Gunmen also fired on the Baghdad office of a major Kurdish party and two senior officials escaped assassination in separate attacks in the north.
The U.S. military put the death toll from the day's Baghdad bombings at 26, saying the number was based on initial reports at the scene. Iraqi officials gave a lower toll — 12 people killed in the bombings and one at the Kurdish office.
Sunni Muslim insurgents have threatened to disrupt the elections, and the five car bombings — four within a span of 90 minutes — underscored the grave threat facing Iraqis at this watershed in their history. U.S. and Iraqi forces have stepped up raids and arrests in Baghdad, Mosul and other troublespots as the elections approach.
Nevertheless, the attacks had little effect on preparations for the Jan. 30 balloting, in which Iraqis will choose a 275-member National Assembly and regional legislatures. At Baghdad airport, Iraqi authorities Wednesday received the largest shipment of ballot boxes and other elections equipment to date.
Elections official Farid Ayar said 90,000 ballot boxes had already been flown to Iraq along with millions of ballots printed mostly in Canada and Australia.
In other developments:
Throughout the morning Wednesday, the routine clatter of big city traffic was punctuated by the crisp sound of distant explosions. U.S. military helicopters rattled low overhead, roaming the bright blue sky for any sign of trouble.
Al Qaeda's branch in Iraq claimed responsibility for the first of the day's blasts, which occurred about 7 a.m. at the Australian Embassy in the capital. A truck packed with explosives blew up outside the concrete barriers in front of the embassy, killing two people and wounding several, including two Australian soldiers.
"A lion of monotheism and faith ... carried out a martyrdom operation nearby the Australian Embassy," the group al Qaeda in Iraq said in an Internet statement. The group is led by Jordanian militant Abu Musab al-Zarqawi, who has allied himself with Osama bin Laden's terror network.
Lt. Col. Steven Boylan, director of the U.S. military press center, said U.S. officials could not explain the discrepancy between the American and Iraqi figures "but we are holding to our numbers."
A half-hour after the embassy blast, another car bomb exploded at a police station next to a hospital in eastern Baghdad. The U.S. military said 18 were killed there, but the Iraqi Interior Ministry put the death toll at six, including a policewoman.
A third car bombing struck at the main gate to an Iraqi military recruiting center located at a disused airport in central Baghdad. Police said the driver told guards he was delivering potatoes and detonated his explosives at the gate, killing three Iraqi soldiers and injuring one American.
The U.S. military also said a car bomb detonated southwest of Baghdad International Airport, killing two Iraqi security guards. The fifth car bomb exploded around noon near a Shiite mosque and a bank in north Baghdad, killing one person and injuring another, police said.
Also in the capital, insurgents in a car fired on an office of the Patriotic Union of Kurdistan, killing one of its members and wounding another, party officials said.
Elsewhere, an Iraqi police officer was killed Wednesday in another car bombing in the largely Shiite city of Hillah south of Baghdad, the Polish military said.
In London, Janusian Security Risk Management, Ltd., confirmed two of his employees — one Briton and one Iraqi — were killed and a third, also a foreigner, was missing after an ambush in Beiji, 155 miles north of Baghdad.
Also in Beiji, gunmen killed two Iraqi policemen and abducted a Japanese engineer, police Lt. Shaalan Allawi said. The engineer's name was not released.
Hiroyuki Hosoda, Japan's top government spokesman, said on Thursday that Tokyo had not heard that a Japanese person had been kidnapped in Iraq but was checking.
"Other information indicates (the victim) was not a Japanese citizen," Hosoda said at a news conference. "We would like to confirm details."
Akira Chiba, assistant press secretary at Japan's Foreign Ministry, said Tokyo had not known of any Japanese engineers working at the power station.
To the north, the dean of the police academy in the Kurdish self-governing region, Maj. Gen. Wirya Maarouf, escaped an assassination attempt when gunmen opened fire on his convoy in the city of Irbil. One bystander was killed and another injured, said police Col. Tharwat AbdulKarim.
Farther north, a roadside bomb exploded in Dahuk near the convoy of provincial Gov. Nejrivan Ahmed, but he was not injured, AbdulKarim said.
U.S. and Iraqi officials had predicted an escalation in violence as the elections approach, with Sunni insurgents seeking to frighten people into staying away from the polls. Sunni clerics have also called for a boycott because of the presence of U.S. and other foreign forces on Iraqi soil.
Although the majority Shiite Muslims and the Kurds are expected to vote in large numbers, officials fear a low turnout among Sunni Arabs may cast doubt on the legitimacy of the new government and sharpen communal tensions among the country's 26 million people.
President Bush called interim President Ghazi al-Yawer on Wednesday to discuss preparations for the elections, including security and how to encourage voter turnout among Sunnis.
Later Wednesday, al-Yawer urged his fellow Sunnis to "reject the grip of terrorism" and take part in the elections. Al-Yawer is among a number of Sunni Arab politicians who have urged Iraqis to reject insurgent threats.
"Voting is a national duty and a right of every Iraqi," al-Yawer said in a message marking the start of the four-day Islamic holiday Eid al-Adha, which begins Thursday. "From now on, the people must be the source of power."