In a separate incident at least seven people were wounded in clashes between US forces and al-Mahdi Army, al-Sadr's militia, who seized a number of police stations in al-Sadr City suburb of Baghdad on Sunday.
Al-Mahdi Army earlier occupied at least three police stations in the area, forcing police forces out of the buildings.
Earlier, Spanish occupation forces killed at least 20 Iraqi demonstrators and injured more than 150 others in the southern city of Najaf, reported Aljazeera's correspondent Muhammad al-Sharif.
Four occupation soldiers from El Salvador were killed and nine wounded near Najaf, said the Spanish Defence Ministry.
About 15,000 of al-Sadr's supporters staged a peaceful protest heading towards the Spanish headquarters in the southern city.
According to al-Sharif, Spanish troops and helicopters opened fire randomly at the protesters, while some witnesses say demonstrators threw stones at military vehicles.
Police stations seized
Al-Sadr's militia, al-Mahdi army, had also taken control of the police stations in Najaf.
Occupation forces also fired at journalists at the scene, including Aljazeera's crew, injuring al-Sharif.
Al-Sadr's office has issued a statement calling on supporters to end their protests, declare jihad and take up arms against occupation forces.
In Amara, another southern city, British occupation soldiers killed an Iraqi and wounded five others during clashes between forces and al-Mahdi Army militiamen, said hospital officials. Iraqi police also fired shots amid demonstrations in central Baghdad, leaving at least two people injured. Protesters were marching towards police headquarters in Saadun Street.
Members of al-Mahdi army
parade in al-Sadr city
And in the northern city of Kirkuk, at least three Iraqi civilians were killed when a car bomb exploded in the centre of the city. Al-Sadr's supporters also took to the streets to show their support.
Two US occupation soldiers were also injured in the blast.
Protesters demanded the release of Mustafa al-Yaqubi, a senior aide to al-Sadr, detained by occupation soldiers on Saturday.
There have been daily protests for the past week over the occupation's suspension of al-Hawza newspaper, a pro-al-Sadr publication which the US occupation authorities said was inciting violence.
Mosques linked to al-Sadr had earlier called for a general strike. Al-Sadr is a fierce opponent of the occupation and US-appointed Iraqi Governing Council. His support is mainly among impoverished Iraqis living in al-Sadr city in Baghdad, formerly known as Saddam City.
On its part, the US-led occupation in Iraq confirmed it had detained al-Yacubi, in connection with the killing of Shia cleric Abd Al-Majid al-Khui last year.
Al-Khui was hacked to death at a Najaf mosque in April, 2003
by a mob which also killed one of his aides. Senior clerics at
the time blamed the killings on a group linked to al-Sadr. Al-Sadr's group denied the charge.
"Coalition forces arrested him yesterday in connection with
Khoei's killing," a senior occupation official told reporters.
He said 25 other warrants had been issued in connection with al-Khui's murder. Thirteen suspects were in custody. He said there was no resistance to the arrest.
Meanwhile, a bomb exploded outside a small mosque in the town of Baquba, north of Baghdad, destroying most of the building and wounding at least one person, witnesses said.
US occupation soldiers are on
high alert in Baghdad
A Reuters television cameraman at the scene said al-Rasul al-Adham mosque, predominantly attended by Shia Muslims, had collapsed from the force of the explosion, which went off after morning prayers when the building was mostly empty.
It is not the first time a mosque has been attacked in the town.
In January, five people were killed and more than 30 wounded when a bomb exploded outside another Shia-dominated mosque in the centre of Baquba as Friday prayers were ending.
The town, which lies about 65km north of Baghdad, is inhabited by Sunni and Shia Muslims and has seen regular unrest over the past year, with fighters who oppose occupation attacking US forces and Iraqi police.
US marines killed
In related news, two US marines were killed in separate attacks west of Baghdad, the US military said in a statement on Sunday.
The army said one marine was killed in action on Saturday and the other died on Sunday from wounds received in a separate engagement.
The military declined to give any further information on the incidents citing security reasons.
The area west of Baghdad, including the town of Falluja, is a centre for anti-occupation activity. Earlier this week, four US contractors driving through the town were killed and their corpses burned and dragged through the streets by a crowd.
The same day, five US soldiers were killed in the area when a roadside bomb detonated under their armoured vehicle.