BAGHDAD, Iraq — Standing side by side, dozens of 12-foot-tall concrete slabs loom over the median of an apartment-lined avenue near the Tigris River here.

The 100-foot-long barrier has sprung up to shield a side street from a potential car bomb. At the entrance to the street, more slabs stand next to a sandbagged guardhouse. A half-dozen guards, all carrying AK-47s and some wearing body armor, patrol the entrance.

This is not the Green Zone, the fortified district the U.S. military established after the fall of Saddam Hussein two years ago.

The Sandi Group, an American company participating in a multimillion-dollar contract to advise the Iraqi government on law enforcement, has fashioned its own green zone across the river, one of more than a hundred such private fortresses, large and small, across the city.

Although the U.S. military provides a safety cordon for the interim Iraqi government, the U.S. and British embassies and large contractors such as Parsons Corp. of Pasadena, Calif., many foreign organizations do business outside its checkpoints.

When the insurgency's campaign of bombings and assassinations cast the city into a state of fear 18 months ago, it was up to the companies to provide their own security, spawning a private fortifications industry. Concrete blast walls, trucked around the capital in sections and hoisted in place by cranes, now surround many of the city's landmark buildings and other streets.

These compounds have angered and aggravated Baghdad residents, cutting them off from the foreigners who came to help them and further obstructing the city's clogged streets.

From a design and city-planning view, the walls are disastrous, said Mohammed Ridha, vice chairman of the architecture department at Baghdad University.

They have destroyed the city's continuity and are beginning to afflict the suburbs too, as homeowners, taking their cue from commercial interests, are blockading their houses, he said.

"When we need to improve the security situation of any building now, we put these blocks, any kind, any height, any amount of them," Ridha said.

The concrete slabs, 4 or more feet wide and up to 18 feet high, are generally lined across an entry street between buildings whose facades complete the perimeter. At some of the hotel complexes, hundreds of slabs are strung together in sweeping arcs that swallow up lanes from major boulevards.

The dull gray blocks are often out of kilter, tilting over curbs or anything else in their way, and usually enclose a few nearby dwellings and businesses.

Dozens of embassies outside the Green Zone have been fortified. Formerly posh hotels humbled by economic sanctions and war have remade themselves as hardened bunkers for news organizations unwilling to be hindered by the Green Zone's crowded checkpoints.

The twice-bombed Baghdad Hotel downtown, the Al Hamra Hotel and the Palestine-Sheraton complex — where the foreign media holed up during the war — are all fortresses today, either built privately or with U.S. funds.

Disgruntled Iraqis also say that the demand for security walls has diverted concrete from construction projects, forcing contractors to supplement the country's historically self-sufficient supply with imports from Syria and Jordan.

At about $600 per slab, even a modest "fort" costs tens of thousands of dollars to build.

Small armies must be recruited to staff the checkpoints 24 hours a day. The Hamra, which has a medium-sized compound, hired about 40 guards, all licensed by the U.S. military or the Iraqi Interior Ministry to carry automatic weapons.

To reach the Hamra, a block off the busy boulevard that leads to Baghdad University, a car negotiates a sharp turn to pull between a guardhouse and a 10-foot-high slab.

Before it can pass, guards with assault rifles must raise a crossing gate and withdraw a tire-ripping "dragon's tooth," a hairbrush of rebar spikes welded to a sewer pipe.

Once past the guardhouse, the car zigzags around more slabs to a second guard, who inspects the chassis for bombs with a mirror.

The hotels, companies and embassies find a ready supply of guards among the 400,000 men put out of work nearly two years ago when occupation authorities disbanded the Iraqi army.

In the regulatory vacuum of postwar Iraq, no city permits or inspections are required to establish a fort. A single property owner or well-heeled tenant can take the initiative. Usually, the added value of the security gives other apartment and business owners an economic incentive to go along.

"The leases of the houses inside this compound are now much more," said Mohammed Sadek Yassin, who owns a building inside the fortress guarded by Sandi Group. "These organizations and others are trying to rent other houses inside."

Inside the city's private blast walls, a kind of normal life goes on beside the comings and goings of foreigners. About 30 Iraqi families still live in the Sandi Group compound, Yassin said. Children play in dirty streets, women in black abayas carry shopping bags home from market and small businesses persist despite the obvious disadvantages.

The security is generally welcomed by Iraqis who live or work inside them, even if not with much cheer.

Outside the walls, that ambivalence hardens into resentment.

"If you ask anyone, they say this is not acceptable to see these barricades in front of my shop, said Adil Mohammed, the manager of a storefront grocery across the street from the Palestine.

Besides harming business, he said, the walls are demoralizing and destroy his city's beauty.

One day, the soldiers and the reporters will go. If they should leave a more peaceful society behind, Baghdad will have thousands of useless slabs to get rid of.

But Ridha, the architecture professor, says Baghdad's blocks could be used effectively in buildings, public landscape and highways, without offending the city's rich heritage.

"It's possible," he said with a quizzical shrug.