New coils of barbed-wire and blast barriers marked checkpoints that caused traffic bottlenecks. U.S. Apache helicopters were in the air over parts of the city where they hadn't been seen before. Gunfire still rang out across the city and some residents said they doubted life would get better.
"Nothing will work. It's too late," said Hashem al-Moussawi, a resident of the Sadr City Shiite enclave who was badly wounded in a bombing in December.
Underlining the dangers ahead, a Wednesday northwest of Baghdad, killing all seven people on board, the U.S. military said. It's the fifth helicopter to go down in Iraq in just over two weeks.
CBS News has learned that the transport helicopter was shot down during what the military called "routine operations."
But before the U.S. could announce the cause of the crash, a jihadi Web site linked to al Qaeda was already declaring victory, reports chief foreign correspondent Lara Logan.
The Web site said, "The Air Defense Division of the Islamic State of Iraq has succeeded in shooting down and completely burning a Chinook helicopter."
"The helicopter was flying and passed over us, then we heard the firing of a missile," said Mohammad al-Janabi, a farmer who was speaking less than half a mile from the wreckage. "The helicopter then turned into a ball of fire. It flew in a circle twice, then it went down."
In other developments:
At checkpoints that seemed to have been thrown up overnight, some of them blocking half the lanes of traffic on wide streets, Iraqi police and army soldiers searched cars at random. Drivers and passengers had to get out and show identity papers.
Adding to the tension, Iraqi Army and police convoys fired rounds into the air above motorists, warning them to make way for the passing forces. The security troops drove over traffic medians and into incoming traffic.
In a sign of just how dangerous the security mission will be, a three-vehicle western security company convoy came under fire near Haifa Street, a Sunni insurgent stronghold just north of the Green Zone.
The security men in the armored cars returned fire and quickly exploded green and white smoke bombs for concealment. Minutes later, they sped away, with the bodyguards in the convoy pelting surrounding autos with water bottles to make them clear the way.