Military operations continued against Mahdi Army forces loyal to Shiite cleric Muqtada al-Sadr, who remained control of streets in Basra, Iraq's main oil hub.
More than 200 people have died in street battles that began Tuesday in Basra and Sadr City, Baghdad's main Shiite district, and spread to other southern cities, reported the Associated Press.
Al-Maliki, who is also a Shiite but belongs to a different political party than the Sadrists, has faced mounting anger due to the military crackdown, which he pledged to continue Thursday.
"We have made up our minds to enter this battle and we will continue until the end. No retreat," he said on Iraqi state television, according to the AP.
Demonstrators in Baghdad carried a coffin with a crossed-out picture of al-Maliki, calling him a "new dictator."
Sadrist lawmakers in Baghdad demanded a halt to the military action. "We call on our brothers in the Iraqi army and the brave national police not to be tools of death in the hands of the new dictatorship," said Sadrist lawmaker Falah Shanshal, quoted the AP.
The escalation of violence threatened to unravel a ceasefire put in place by al-Sadr in August.
The demonstrating Sadrists said U.S. and Iraqi forces are taking advantage of the truce to crack down on the movement.
U.S. commanders have insisted the fight is being led by the Iraqi government and was not against al-Sadr's movement but breakaway factions believed to be trained and funded by Iran, reported the AP.
Meanwhile, President Bush praised al-Maliki for the campaign against the militias in a speech Thursday.
"Prime Minister Maliki's bold decision, and it was a bold decision, to go after the illegal groups in Basra shows his leadership and his commitment to enforce the law in an even-handed manner," the president said at the National Museum of the U.S. Air Force in Dayton, Ohio.
He credited the surge for allowing Iraqi security forces to make progress and lawmakers to pass critical legislation.