The prime minister was not at the building in Baghdad's Mansour neighbourhood on Friday at the time of the attack, which wounded four people, party official Ayad al-Nedawi said.
He said the bomber detonated the car near a checkpoint about 25 metres from the party's offices, which used to be al-Jaafari's house.
Interior Ministry administrative affairs undersecretary police Major General Adnan al-Assadi had left the party offices about a half hour before the explosion, al-Nedawi said. It was not immediately clear why he was there or if he was the target of the attack.
The person who died was a civilian who lived next door to the offices and the four wounded were armed party members guarding the compound, said al-Nedawi, who initially said the victim who was killed was a guard and only one other person was wounded.
The attack occurred at about 12.30pm (0830 GMT), police First Lieutenant Thair Mahmoud said.
Official's cousin killed
In the al-Amiriya district west of Baghdad, attackers on Friday gunned down Tahir al-Rubie, cousin to Iraq's National Security Adviser Muwafaq al-Rubie, and one of Tahir al-Rubie's sons, sources at al-Rubie's office told Aljazeera.
The attack occurred in a private shop operated by Tahir al-Rubie, the sources said.
The national security adviser denounced the killing as "cowardly and treacherous".
Al-Sistani aide shot
A senior aide to Iraq's Shia spiritual leader Grand Ayat Allah Ali al-Sistani was shot dead in Baghdad on his way to Friday prayers, security sources and al-Sistani's office said.
The attack occurred in the tense
Haifa street area
Shaikh Kamaleddin al-Ghuraifi was killed along with two of his companions when his car was attacked in the tense Haifa street area as he headed to the Al-Duriyeen mosque, said a Defence Ministry source, adding that three others were wounded.
Al-Sistani's office in Najaf confirmed al-Ghuraifi's killing and said he was one of the cleric's senior aides in Baghdad.
Another al-Sistani aide, Samir al-Baghdadi, was killed in the capital a week ago, and in January al-Sistani's representative in Salman Pak-Madain, a restive area southeast of the capital, was killed.
Meanwhile, Iraqi media sources told Aljazeera that scores of Iraqis were killed and injured in rocket-propelled grenade and machine-gun attacks on the Musaab bin Umair mosque in the northern town of Talafar.
The sources said the attack took place during Friday prayers. The attackers fled without being identified.
Blaze cuts water
A fire erupted on Friday in a transformer room that feeds electricity to a main water plant near Baghdad, shutting it down and leaving millions of residents in the capital without water, officials said.
The blaze, whose cause was under investigation, came a day after Baghdad's mayor decried the capital's crumbling infrastructure and its inability to supply enough clean water to residents, threatening to resign if the Iraqi government won't provide more money.
Millions of residents in the capital
are without drinking water
Friday's fire began at about 7am (0300 GMT) and affected the Karkh water station in Tarmiyah, which serves northern and western Baghdad, officials said.
The water project's director, Jassim Mohammed, said he thought the fire resulted from an explosion carried out by fighters.
He said at least three days were required to get spare parts and repair the damage, which has halted all water distribution from the plant.
A municipal official, however, said the blaze was still under investigation.
Loud explosions can occur when a transformer blows.
"The reason remains unknown. It's not clear if it was an explosion (caused by fighters) or a technical problem," said the official, who spoke on condition of anonymity because of the sensitivity of the information.
The water stoppage in several neighbourhoods will continue until the problem is fixed, the official said.
The US military confirmed the incident, said Sergeant First Class David Abrams, a spokesman for Task Force Baghdad.
"We verified with engineers on the site that it was a blown transformer, but not an attack"
Seargeant First Class David Abrams,
Task Force Baghdad spokesman
"We verified with engineers on the site that it was a blown transformer, but not an attack." Abrams could not immediately provide information on how many residents were affected.
Mayor Alaa Mahmoud al-Timimi's threat to resign was an indication of the daily misery that Baghdad's 6.45 million people still endure more than two years after the US-led invasion. They are wracked not only by unrelenting bombings and kidnappings, but by serious shortages in water, electricity and fuel.
"It's useless for any official to stay in office without the means to accomplish his job," al-Timimi told reporters on Thursday.
Al-Timimi is seeking $1.5 billion for Baghdad in 2005 but so far has received only $85 million, said his spokesman, Ameer Ali Hasson.