Mr Maliki accused members of executed dictator Saddam Hussein's regime of fomenting the violence and said Iraq's security forces had now taken control of the city, still smouldering from a night of bitter fighting.
The indefinite curfew took effect from 11 am (0700 GMT) and applies to people and vehicles, Iraqi state television quoted Mr Maliki as saying during a visit to Karbala.
Security force reinforcements had been sent to the city, 110 kilometres south of Baghdad, and were now in control, Mr Maliki said.
A correspondent with news agency AFP in Karbala said battles between gunmen and police which broke out on Tuesday when the streets were flooded with Shia pilgrims raged through the night but died away early on Wednesday.
Gunmen were seen at dawn encamped in a square near the old city, which houses shrines to two Shiite saints, Imam Hussein and Imam Abbas, one of Shiite's holiest sites and the focus of Tuesday's pilgrimage.
Several buildings were burned down during the night while ambulances were smashed and a police checkpoint destroyed, the correspondent said.
Medical officials said at least 52 people had been killed. Around 300 people were wounded, with at least 60 of them sent to hospitals in the nearby city of Najaf for treatment.
Maliki under fire
Mr Maliki, who is under fire at home and abroad for failing to bring stability to Iraq, blamed the bloodletting on "criminal outlaw gangs from the remnants of the Saddam Hussein regime."
"The terror acts implemented by these hired groups led to the martyrdom and the injury of a number of pilgrims and damage to public property," he said in the statement.
But local officials said the Mahdi Army Shiite militia of anti-American Shiite cleric Moqtada al-Sadr, who have established a stronghold in Karbala, were behind the fighting.
A security official said a senior Sadrist, Hamid Gannoosh — a member of Karbala provincial council — had been arrested in Karbala on Wednesday for his alleged role in the violence.
Sadr on Wednesday denied that his militia was involved.
"What happened in Karbala was not a fight between the Mahdi Army and the Iraqi government. It was rather a fight between civilians and the government and the Mahdi Army had nothing to do with it," Sadr's spokesman Sheikh Ahmed al-Sheibani quoted the cleric as saying.
Tuesday's clashes sent hundreds of thousands of pilgrims fleeing in panic.
They had been streaming into the city from across the country to celebrate the birth anniversary of eighth century Imam Mohammed al-Mahdi.
Shiites believe Imam Mahdi, the 12th Shiite imam, disappeared centuries ago from the northern Iraqi town of Samarra but will return one day to save the world.
Karbala police chief Brigadier General Hamid Raad Shaker said clashes erupted when gunmen shot at police who returned fire, and that several mortar rounds struck near the Imam Hussein mausoleum.
He did not offer details on the identities of the gunmen, but Karbala is known to be a bastion of the Mahdi Army, while many police are loyal to the powerful Shiite party, the Supreme Iraqi Islamic Council (SIIC).
The Sadrists and SIIC are battling each other for control of towns and cities in Iraq's mainly Shiite south, adding to the overlapping conflicts of an insurgency against US-led forces and brutal sectarian bloodletting.