Colonel Salam al-Maamuri, head of the US-trained Iraqi police special intervention team, died along with his deputy in a blast that left at least eight other officers injured, Iraqi police said on Friday.
"He was transferred to hospital after the bomb went off, but he died on the way," said Lieutenant Kadhem Shamari, as US troops arrived to seal off the brigade's headquarters in the southern city of Hilla.
Doctor Haidar Timimi at Hilla's main hospital confirmed the death and said eight wounded officers were receiving treatment.
Lieutenant-Colonel Michelle Martin-Hing, US military spokeswoman, said the bomb appeared to have been packed with ball bearings to render it more lethal.
Al-Maamuri had previously survived several assassination attempts.
His squad was set up with US support to counter fighters opposed to the US presence in the country in an area south of Baghdad, including a region that became notorious as the Triangle of Death.
Also on Friday, armed men attacked a group of Shia women picking vegetables in a field, killing six adults and two young girls and kidnapping two teenagers.
The attack took place in fields outside Saifiya, a mixed Sunni-Shia village on the southern outskirts of Baghdad.
Most residents have already left to escape violence, the Sunnis going to the nearby town of Madain, the Shia to neighbouring Suwayrah.
The shooting was one of the deadliest single attacks specifically targeting women in Iraq's long history of sectarian violence.
Police said they suspected Sunni fighters seeking to intimidate Shia into fleeing the area.
The women were gathering vegetables when armed men pulled up in two cars and surrounded the field.
They opened fire, killing six women and two girls aged about four or five years old. The attackers then forced two teenage girls into their vehicles and escaped.
In other separate incidents, at least 10 other Iraqi civilians were killed in violence on Friday, while a US soldier was killed by a roadside bomb in northern Iraq, the 45th American death in Iraq this month.
A new video on the internet has hinted at possible splits between Iraqi Sunni fighters and al-Qaeda in Iraq.
A man claiming to be an Iraqi Sunni fighter released a message asking al-Qaeda leader Osama bin Laden to replace the leader of al-Qaeda in Iraq because of its attacks against Sunni clerics.
Abu Osama al-Iraq said: "They planted explosives in the houses, the hospitals and the schools and even the electric transformers. If we lose [Sunni support] we will be an easy hunt for the Crusaders, the occupiers and their agents, the Shia."
He asked for bin Laden to remove Aby Ayyub al-Masri, al-Qaeda's leader in Iraq, and replace him with an Iraqi.
In other news, Shia cleric Moqtada al-Sadr has ordered his Mahdi Army militia not to take part in the sectarian violence and told those who had killed Iraqis to "repent".
"There are rumours that there are groups or persons from the Mahdi Army attacking the Iraqi people with no right to do so," al-Sadr said in a statement distributed by his office in the city of Najaf on Friday.
"It is not proved so far but, if proved, I will declare their names and will renounce them with no fear or hesitation."
Al-Sadr has ordered his loyalists
to refrain from sectarian killings
The US military have said that Shia militias are the biggest single threat to the stability of Iraq and they are waiting to launch an operation to clear Sadr City of armed men.
Meanwhile, Nuri al-Maliki, Iraq's prime minister, has been urged to open an investigation under his personal leadership to identify the perpetrators of an attack on a new TV channel.
The Journalistic Freedoms Observatory in Iraq made the appeal after the deaths of 11 employees of Al-Shaabiya TV, including its general manager and his deputy.
The observatory said in a statement the attack was carried out by an armed group wearing Iraqi police uniforms.