Itay Livni walked to work on Tuesday from his East Village apartment, just like he always does. The stroll through Chinatown, watching the produce trucks unload their goods, gives him a chance to clear his head, and also to experience a bit of the real world before entering a land of numbers and probabilities. “Once you enter the building, you don’t see anything,” he says. Well, not on a normal day, anyway.

Livni, an options trader on the American Stock Exchange, works for Tahoe Trading, a market maker based on Rector Street, a couple of blocks south of the World Trade Center. The building, an old and ornate stone skyscraper, also houses the thestreet.com.

Livni got to his office building at 8.45am, and sat down on the steps to finish off a final cigarette before starting his day. He was just taking his final drags when he heard a big boom. He didn’t think much of it: he thought it was probably just a heavy truck going over metal plates on the street. He dismissed the natural inclination to think it might be a bomb.

That thought was rapidly dismissed, however, when he looked around the corner. The air was full of paper, “like the most massive ticker-tape parade ever,” he says. At that point, he knew something was seriously amiss, and ran up the steps to his building to find out what was going on.

When Livni got out of the lift on the 17th floor, his fellow traders were already clustered around the televisions. They knew at this point that a plane had flown into the World Trade Centre. The view out of the window was of the south tower, but it was easy to see the plume of smoke rising behind it. No one knew yet that this was a terrorist attack. “We thought a pilot had gone astray,” Livni recalls.

As the traders started putting on their jackets, the market started tanking. Livni had his eye on the Dow Jones futures, which plunged 150 points immediately, and then recovered to 90 down, and then 32 down. “At that point, I wanted to sell,” recalls Livni, but before anybody could do anything, they were back down 150 points again.

Livni remembers thinking that he was in for a good day: volatility had gone wild. But then, as he was heading out of the building, the second plane hit, and the earth shook. People were running everywhere: mainly away from the World Trade Centre and its falling debris, but also out of the Rector Street building. Livni had a better idea: he went straight back into the building, and back to the 17th floor. “I felt that it was the safest place,” he says. “Two Rector Street doesn’t hit anybody’s maps.” Three people were left in Tahoe’s offices.

At this point, there was no doubt that New York was under a terrorist attack of unprecedented magnitude. Livni started phoning the traders who were already on the floor of the exchange, telling them to go home. He alternated between watching the burning towers on the television screens, and looking out of the windows to see them in real life.

Still, few people imagined that the towers could collapse. “It seemed like it was all over,” says Livni. He asked his boss if he could smoke a cigarette, and lit up, watching the tallest building in New York slowly burn in front of his eyes.

Livni then booted up his computer, and started computing the company’s risk positions. Just as he was about to tell his boss the result, he looked over and heard him say “holy shit”. A wall of debris flew towards the window, and everybody in the office dived under the nearest desk as the earth shook and the lights went briefly out.

Ten seconds later, all three of them were belting down the stairway, to the safer second floor of the building, which coincidentally housed another clearing house. Others just ran as far away from the World Trade Centre as possible, but when Livni saw them doing that, he says, “that’s the exact reason I didn’t run. People like that create pandemonium.”

Indeed, very quickly people started coming in off the street, dazed and covered in soot. One man, after being dusted off, immediately asked where the television was: a crowd was quickly developing around it. There were no major injuries, just a lot of coughing and minor cuts and abrasions. One member of a rescue team was hurt more badly, limping up the stairs, upset that he couldn’t help out any more.

Then the second tower collapsed. “This time the building really, really shook,” says Livni, who dived under his second desk of the morning. The assembled group decided it was best to stay put, and frantically tried to get phone lines, either from the office or on their cell phones, with little luck.

“Looking out the windows, you could not see a thing,” says Livni. “It was darker than night.” Occasionally there was a rumble — more often than not from a fighter jet overhead — which caused dozens of people to duck.

Finally, one of the professional rescue teams arrived, and told everybody to grab as many paper towels as they could. As some of the debris cleared from the air, a group of maybe 100 people gathered in the building’s lobby. One Japanese man was in severe shock, with injuries to his legs, which had been tied together. While one of Livni’s colleagues tried to get the man’s wife on his cellphone, Livni held his hand and told him he was going to be fine. He couldn’t do much else: “Every time you patted him, loads of soot went in the air,” says Livni, and there was too much of that already.

The rescuer addressed the crowd, and told them it was really bad out there. They were to form a human chain, and walk out single file; women were told not to take off their heels. A man called Bob led the way, followed by everybody else, each with some article of clothing or towel over his or her face.

Walking south, towards the Staten Island ferry, more and more people started joining the chain, and the air started to clear up. By the time they got to the park, everybody knew they were going to be safe. “People were so helpful and nice to each other,” says Livni.

Eventually, Livni joined the dust-covered exodus of people walking up the FDR Drive, which goes up the east side of Manhattan. Volunteers were handing out water, and when the crowd — now numbering many thousands — reached the housing projects at the bottom of the island, they were met by locals offering their bathrooms and telephones for anybody who needed them.