Soldiers and police were deployed on the main eastern highway leading to the Shi'ite district of Al-Sadr City, and new checkpoints have been set up in at least five districts of the capital.
U.S. troops are participating in the operation, with the intent of clearing neighborhoods of militants and illegal weapons.
Latest Deadly Attack In Baghdad
This comes after the deadliest single bomb attack since the U.S.-led invasion of 2003. On February 3, a suicide truck bomb killed some 130 people in a mainly Shi'ite area of Baghdad and wounded more than 300 others.
Ordinary Baghdadis are weary of the sectarian violence that has killed some 1,000 Iraqis in the last week.
"We hope the security crackdown is serious [and will] benefit the Iraqi people, because Iraqis, from all sects, have been suffering for 35 years from injustice, oppression, starvation, and other things," one man told Reuters.
"We urge the officials in the name of God to speed up the implementation of the [security] plan and help people who suffered injustice," another said.
The security sweep was promised by Shi'ite Prime Minister Nuri al-Maliki last month, while U.S. President George W. Bush has committed 17,000 more troops for Baghdad to help.
'Political, More Than Military Issue'
But this is the third bid to boost security in the capital in nine months. Will the new drive succeed?
London-based journalist and Middle East expert Alireza Nourizadeh tells RFE/RL he has little hope, and says the problems of Iraq lie in the sectarian animosity between the biggest political groupings now ruling the country.
He says it is very unlikely al-Maliki will turn against the armed groups of his own Al-Da'wah Party or against the Shi'ite Imam al-Mahdi Army, which is loyal to radical cleric Muqtada al-Sadr. The majority of Iraqi Army conscripts are also Shi'a, he notes.
"The problem in Iraq is a political problem rather than a security or military problem," Nourizadeh says. "It is a political problem and I think President Bush's plan should be implemented with the will and intention of people like Mr. Nuri al-Maliki."
In a sign of the size of the task ahead, there was fresh violence in the capital today, when at least 24 people were killed in separate car bombings
The Imam Al-Mahdi Army on parade (epa)
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THE COMPLETE PICTURE: Click on the image to view RFE/RL's complete coverage of events in Iraq and that country's ongoing transition.