While experts say that people have survived for more than six days in other disasters, the fact that no one has been found alive at the Trade Center since Wednesday makes it difficult to maintain much hope, Mr. Giuliani said.

"Everybody should realize the chances decrease substantially as time goes by," he said. "We want everyone to prepare themselves."

The city is now listing 5,422 people missing and 201 confirmed dead, 135 of those identified.

Even New York Fire Commissioner Tom Von Essen, whose crews have led the rescue work, seemed resigned.

"I'm becoming less optimistic," Mr. Von Essen said. "The firefighters, the police officers ... are devastated, but not defeated."

Two New York City police officers who had just returned from the disaster site said it was impossible to believe that anyone could have survived.

"They're just pulling out bits and pieces," said one. Rescue crews continued their exhausting work, having hauled away nearly 40,000 tons of twisted steel and shattered concrete so far.

They've been on the job since shortly after terrorists steered a pair of airliners into the World Trade Center last Tuesday morning. That's when much of New York closed down, including the stock markets, which handle the business of America.

On Monday, though, Wall Street reopened and the streets were filled with well-dressed professionals who carried small American flags and wore surgical masks amid the smoke and dust.

"You're of course apprehensive, but the bottom line is we have to get the exchange open and we have to get back to normalcy," said Tom Cook, a broker who returned to work with the Stars and Stripes draped over his shoulders.

With some subway lines still shut down near the Trade Center and other commuter lines diverted, Monday's commute was more difficult than usual.

Traffic jammed the Holland and Lincoln tunnels, stretching the trip by an hour or more.

With traffic banned below Canal Street, commuters depended upon a patchwork of subways and ferries to reach lower Manhattan.

Somesh Rao, who works for Goldman Sachs, stepped off an unfamiliar train at an unknown station and stood blinking at the scene around him, hoping for a landmark.

"It's like a completely new place," Mr. Rao said.

"A lot of visual things you depend on [for navigation] are gone – obviously, the World Trade Center – and with the smoke, none of the buildings looks familiar. The streets are blocked off; it's confusing."

The Transit Authority provided a free ferry from Brooklyn, the first since the Brooklyn Bridge was completed in 1883. The bridge remained closed Monday to all but foot traffic.

The Staten Island Ferry resumed operations for the first time since the attack.

On board, Albert Bertone stared somberly across the water. Gray smoke from the twin towers' rubble drifted toward the Statue of Liberty. Ferry riders could smell it for the entire half-hour trip.

"I'm fine. I'm actually fine," said Mr. Bertone, field supervisor for a security agency. "You got to get on with living."

Christine Acosta said she was glad that vehicles were prohibited from the ferry until further notice.

"Car bombs," she said.

When the terrorists struck, the ferry shut down, Mrs. Acosta said. So she walked most of the way home via Brooklyn.

"Twenty-two miles, in heels."

Though New Yorkers returned to work in significant numbers – Mr. Giuliani estimated that 75 to 80 percent of the workforce was on the job – this was far from a typical workday.

Security was tight. The only vehicles on the streets were police cars, National Guard trucks and utility vehicles. Troops checked every person trying to enter certain areas.

Dave Rivera couldn't get in. Mr. Rivera, who works for an oil and gas development company, carried an expired driver's license.

"I don't know what to do," he said. "I can't even call my office."

Mr. Rivera said he has been struggling since the attack.

"I saw everything when it happened. It finally sunk in on Thursday, and things haven't been the same since," he said. "I can't eat, I can't sleep. Every time I close my eyes, I get the shakes."

Many of the buildings in the Wall Street area were pockmarked by the force of the explosions and the collapse of the twin towers.

Stock trader Craig A. Brandwein said the dust and dirt were the only problem in his building, though some neighbors were still without power and phones.

"It's strange, a little uneasy," Mr. Brandwein said. "But we've got to show them they can't get to us. This is the hardest part."

The Rev. Paul Smith, pastor of First Presbyterian Church in Brooklyn Heights, came to Wall Street to lead a prayer service at a friend's investment house.

"He didn't want to begin the first day back without giving thanks to God," Mr. Smith said.

The smoke lingered along Wall Street, pungent and strong.

"I'm not happy about it," said Joe Gliozzo of his return to work. "You don't know what you're breathing, you know?"

At Zeytuna, a deli and restaurant, customers stood in line for breakfast.

"There's been big lines since 7 in the morning," said manager Jamel Feknous.

The only downside: Just a handful of his workers showed up.

"Twenty to 25 percent of my staff is here," he said. "I've been calling them all week. They're scared, and I think most are looking for other jobs. They think it's very risky being down here."

Mr. Giuliani doesn't buy that.

"Civilizations have been attacked, democracies have been attacked, and they've had the strength and the will and the courage to deal with it," he said, referring to London in World War II but hoping the lesson stuck with New Yorkers.

And in many ways, it did.

When the lawyers at his firm arrived Monday, Paul Carter said, the talk was about the attack.

"But as the day wore on," he said, "people started talking about their cases."

Brian Kelly drove into Manhattan from his home in Rockland County, parked three miles from his business – A.J. Kelley's Bar on Stone Street – and rode his bike the rest of the way with the daily bread lashed to his handlebars.

"I still gotta feed my family," Mr. Kelly said.

It was far from a normal day in New York but closer than it has been for almost a week.

Trains rolled in from the boroughs and the suburbs. Ferries plied their routes.

At the ferry terminals, though, Red Cross workers met commuters with surgical masks and leaflets on dealing with trauma. Grief, flashbacks, guilt and anger are common reactions, they warned. So is the feeling of being overwhelmed.

"Whenever possible remember that you are still free and that there is still beauty in the world," the papers counseled.

The advice didn't seem to work for a 29-year-old who identified herself only as Susan. She walked smoke-filled Whitehall Street, her hair still wet from her morning shower, her face full of pain.

"It's absolutely devastating. It's horrible," she said. "All those people. I've been crying for six days."