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, aWednesday 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.
In parts of the city only a few shops were open, a reminder of the fear people have of a repeat of the car and suicide bomb attacks that have hit Baghdad with regularity in recent weeks.
Lines outside petrol stations stretched for more than a mile in the city's worst fuel crisis in months.
Gunfire rang out across the capital and the wail of police and ambulance sirens seemed incessant. The buzz of low flying U.S. helicopters and growl of fighter jets was nonstop above a new crop of posters and billboards speaking of Baghdad's struggle.
"Our streets are deserted and our blood is fair game," declared one that showed an empty street strewn with debris from a bombing.
Another billboard showed a young man weeping because he had not reported suspicious activity to authorities. "I should have done the right thing," he says.
Still another billboard message implored: "Be a hero and report suspicious behavior."
Most taxi drivers were refusing to take passengers headed to areas dominated by the Muslim sect other than their own. Minibus drivers were demanding passengers prove that they lived in the region where they wanted to travel.
The capital's streets became nearly deserted well before nightfall, a surprising sign of genuine fear among a population that has lived through wars for much of the past 25 years. Those with money to spare, residents say, are stocking up on fuel to power generators and on basic foodstuffs like flour, cereals and potatoes.
With electricity only available about two hours a day in much of the city, residents also were buying candles and lanterns. The rickety stands of some outdoor food markets, a favorite target for suicide bombers, stood empty.
"We live hand-to-mouth and don't have money to stock up on anything," said Ibrahim Mohammed, a 78-year-old retired engineer from Azamiya. The predominantly Sunni area in north Baghdad was likely to be high on the list of targets in the Baghdad security plan.
"No one goes to work anymore," Mohammed said.
Maj. Gen. William B. Caldwell, the top U.S. military spokesman in Iraq, said Wednesday, that the much-awaited Baghdad security operation was finally under way. "The implementation of the (Iraqi) prime minister's plan has already begun and will be fully implemented at a later date, having all the parts and pieces that he wants," he told a news briefing.
The operation is the third attempt by Prime Minister Nouri al-Maliki and his U.S. backers to pacify Baghdad since the Shiite leader came to office in May. The operation, which will involve about 90,000 Iraqi and American troops, was seen by many as a last chance to curb Iraq's sectarian war.
"If this security plan is the same as those we had before, with checkpoints delaying the traffic for hours, then I can tell you now that it will be a failure," said Murtadha Mahdi, a 35-year-old unemployed father of two who lives in Hurriyah, now a predominantly Shiite district in northern of Baghdad that saw some of the worst sectarian fighting late last year.
The current security sweep in Baghdad began nearly a year after the city became the main battlefield of sectarian violence after al Qaeda militants bombed a major Shiite shrine north of Baghdad.
Thousands have since died in the capital, victims of Shiite militiamen or suicide bombings blamed on Sunni militants. Thousands more have been displaced from their homes.