NEW ORLEANS – More than 1.2 million people in metropolitan New Orleans were warned to get out yesterday as 140-mph Hurricane Ivan churned toward the Gulf Coast, threatening to submerge this below-sea-level city in what could be the most disastrous storm to hit in nearly 40 years.

Residents streamed inland in bumper-to-bumper traffic in an agonizingly slow exodus amid dire warnings that Ivan could overwhelm New Orleans with up to 20 feet of filthy, chemical-polluted water.

About three-quarters of a million more people along the coast in Florida, Mississippi, and Alabama were also told to evacuate.

Forecasters said Ivan, a killer blamed for at least 68 deaths in the Caribbean, could reach 160 mph and strengthen to Category 5, the highest level, by the time it blows ashore as early as tomorrow somewhere along the Gulf Coast.

“Hopefully the house will still be here when we get back,” said Tara Chandra, a doctor at Tulane University in New Orleans who packed up his car, moved plants indoors, and tried to book a Houston hotel room. Mr. Chandra said he wanted to ride out the storm, but his wife wanted to evacuate: “All the news reports are kind of freaking her out.”

At nearly 200 miles wide, Ivan could cause significant damage no matter where it strikes. Officials ordered or strongly urged an estimated 1.9 million people in four states to flee to higher ground.

“I beg people on the coast: Do not ride this storm out,” Mississippi’s governor, Haley Barbour, said, urging people in other parts of the state to open their homes to relatives, friends, and co-workers. As of 5 p.m., Ivan was about 370 miles southeast of the mouth of the Mississippi River, moving northwest at about 9 mph.

The National Hurricane Center in Miami posted a hurricane warning for about a 300-mile swath from Apalachicola in the Florida Panhandle to New Orleans and Grand Isle in Louisiana.

Forecasters said Ivan could bring a coastal storm surge of 10 to 16 feet, topped by large, battering waves.

New Orleans, the nation’s largest city below sea level, is particularly vulnerable to flooding, and Mayor Ray Nagin was among the first to urge residents to get out while they can. The city’s Louis Armstrong Airport was ordered closed last night.

Up to 10 feet below sea level in spots, New Orleans is a bowl-shaped depression that sits between the half mile-wide Mississippi River and Rhode Island-size Lake Pontchartrain, relying on a system of levees, canals, and huge pumps to keep dry.

The city has not taken a major direct hit since Betsy in 1965, when an 8- to 10-foot storm surge submerged parts of the city in 7 feet of water. Betsy, a Category 3 storm, was blamed for 74 deaths in Louisiana, Mississippi, and Florida.

Experts said Ivan could be worse, sending water pouring over the levees, flooding to the rooftops, and turning streets into a toxic brew of raw sewage, gas, and chemicals from nearby refineries. The mayor said that he would “aggressively recommend” people evacuate, but that it would be difficult to order them to do so, because at least 100,000 in the city rely on public transportation and would have no way to leave.

In addition, he said 10,000 people were in town for conventions, and there was nowhere for many of them to go except high floors in their hotels.

By midday yesterday, traffic on Interstate 10, the major route out of New Orleans, was at a near standstill, and state police turned the interstate west of the city into a one-way route out. U.S. Highway 59, the old major route between New Orleans and Baton Rouge, also was jammed.

In the French Quarter, businesses put up plywood and closed their shutters. A few people were still hanging out at Cafe du Monde, a favorite spot for French roast coffee and beignets, and a man playing a trombone outside had a box full of tips.

“They said get out, but I can’t change my flight, so I figure I might as well enjoy myself,” said George Senton, of Newark, N.J., who listened to the music. “At least I’ll have had some good coffee and some good music before it gets me.”

Tourist Dee Barkhart, a court reporter from Baltimore, was drinking hurricane punches at Pat O’Brien’s bar.

“I looked into earlier flights, but they were hundreds of dollars more and I wasn’t sure I could switch flights,” she said. “I figure I’m happier sitting here drinking hurricanes than sitting at the airport worrying about them.”

But Ms. Barkhart’s drinks would have to be for the road. The bar planned to close by nightfall.

Elsewhere along the Gulf Coast, thousands of residents, gamblers, and tourists crowded northbound roads. Motels were booked as far north as Jackson, Miss., and Montgomery, Ala.

Mississippi regulators ordered a dozen casinos along the state’s 75-milelong coast to close at noon yesterday, but many gamblers pumped coins into the slot machines right up to closing.

“I don’t worry about what’s going to happen tomorrow. We can’t control it anyway,” said Ed Bak of Fairfield, Ohio, who dropped quarters into a machine at the President Casino.

Alabama’s governor, Bob Riley, ordered the evacuation of coastal resorts. “This is a serious storm that requires serious action to get people out of harm’s way,” he said.

Along Florida’s Panhandle, the sounds of saws and drills filled the air as people put up boards to protect their homes and businesses.

“We are just hoping to still be here,” said Matt Claxton, an assistant manager of a Perdido Key seafood restaurant as workers brought the patio furniture inside.