The only time I ever saw devastation on the scale of the ruined World Trade Centre was in the Armenian city of Spitak after the 1988 earthquake. There I recall the deathly quiet and shocking inactivity of a people paralysed by grief. In contrast, downtown Manhattan is a hellish place of noise and activity, dominated by a spirit of determination and defiance.
At dawn on Saturday, despite severe restrictions on the international media, I was escorted into the heart of Ground Zero by a New York Police captain. We approach via the southern tip of Manhattan, passing road blocks manned by National Guard soldiers, and walk north along West Side Highway. Giant Mack trucks loaded with warped steel girders grind their way past us.
We have to step over twisted lines of cables from emergency generators snaking along the six-lane highway and across a new cycle track covered in deep, yellow mud. The highway is blocked by a 40 ft wall of rubble where the pedestrian bridge collapsed.
So wide was this busy walkway which linked the World Trade Centre with the World Financial Centre, a glittering complex of ultra-modern office towers which now looks as if a giant paring knife ran down one corner, that it housed the popular New York orchid fair every year.
At the back of the financial centre, the enormous glass roof of the winter garden has been smashed, the glass cascading down on the famous giant palm trees from the Mojave desert. The boats of the Manhattan Yacht Club have gone from the little harbour and the half-dozen big cruisers which hosted summer champagne parties have been replaced by tugs and barges.
Back across West Side highway, we walk by the wrecked Marriott Hotel and a fire-blackened mini-skyscraper with a huge torn Stars and Stripes flapping from charred scaffolding.
We turn east along narrow debris-strewn and shuttered streets where financial workers patronised dozens of little businesses like Miami Subs, Burger King and Liberty Deli. Instead of the usual pretzel stand there is an asthma booth.
A notice in the dust-covered window of Gino's Pizza Parlour says: "In God we trust, United we stand." An Irish flag hangs from the boarded-up O'Hara's pub. Knee-deep drifts of torn paper cling to the metal railings of a zig-zag fire escape.
In Church Street we are finally confronted with the full awesome panorama of America's biggest crime scene yet. We watch in silence. My escort, who lost 30 of his NYPD comrades, is visibly angry. "Bin Laden is a dead man walking," he says. "We'll get him for this."
All that is left of the 1,350-ft towers is a massive jagged bowl of concrete, steel and debris five storeys high, over which lines of men in safety helmets labour like ants. A giant steel trellis sticks up like the skeleton of a cathedral. Orange cranes loom overhead, draped with American flags.
At ground level, volunteers pass white buckets of debris hand to hand and workers sift their contents for body parts which will be identified by DNA tests. Dumper trucks as small as golf carts climb high to remove concrete lumps and, occasionally, bodies which are laid out in green plastic baths and driven to the food court in the Financial Centre. Sniffer dogs with illuminated collars nose into crevices but hope of finding anyone alive has died.
Only 100 bodies have been found, one of a flight attendant with her hands bound. Almost 5,000 people who occupied 20 million sq ft of office space may never be located, their bodies smashed and entombed along with office furniture, computers and 11.8 tonnes of gold, kept in the vaults to settle trades in futures contracts.
In the dangerous epicentre, steam-fitters, welders and ironworkers burn through twisted steel with acetylene cutters.
"When the rubble starts to shake, everybody runs, but not the ironworkers," says the police captain admiringly, as a despatcher's voice calling for more body bags crackles out on his two-way radio.
Across from the towers, the facades of the Millennium Hilton, the Century 21 discount store and Brooks Brothers store are smashed. The wide street has become an open food court where volunteers hand out McDonald's burgers, Coca-Colas and donated clothing near a supply truck plastered with the words, "God Bless America".
Around the far side of this frenetic scene of activity, the 10-storey ruins of the Number 7 tower, the third to collapse, block Vesey Street. They are still burning despite Thursday's torrential rain, emitting white acrid clouds which drift westwards over Wall Street. Jets of water pour down from three sides.
Back across West Side Highway, dozens of burned-out cars have been piled high as if in a wrecker's yard. Five flattened fire trucks pulled from the wreckage testify to the deadly impact of the falling masonry. The 15-screen United Artists cinema, Lili's Noodle Shop, Appleby's Restaurant, the Tex Mex, and many other popular eating places, are all shuttered, perhaps never to open again.
Exhausted workers lie on the mezzanine floor of the empty Embassy Suites Hotel. Beside it on an acre site stands the gable end of a ruined 19th-century Co Mayo cottage, shipped from Ireland and erected just in the last two weeks as the centrepiece of an Irish Famine memorial. When completed it will take its place among the other memorials in Battery Park City; to Native Americans, to the fallen of the NYPD, to lost merchant marines, to the casualties of war, to Holocaust victims, and, most certainly one day, to the victims of the first air attack on the American mainland.
The World Trade Centre occupied only 16 acres, but several square miles east of Broadway is a dead zone, with rows of empty apartment blocks without electricity or gas, as silent as the surviving buildings in Spitak.
Con Edison Power Company says it must lay 19 miles of new cable before the lights can go back on. In the northern corner is the deserted and locked 43-storey building containing my office and apartment, which I had to evacuate on Tuesday evening carrying only my laptop computer and a few clothes. I was not permitted to re-enter it; residents may only return, briefly, under armed escort, from a special point upriver, to rescue their pets.
The route north from the closed zone lies along West Side Highway. At the checkpoint, a few dozen people stand applauding every vehicle conveying rescue workers in and out, their hands outstretched with bottled water, fruit and confectionery. Firefighters will forever be the heroes of New York's blackest hour.
In 150 years the New York Fire Service lost 700 men; in less than 150 minutes on Tuesday it lost 350. So many volunteers came to south Manhattan to help that many have been turned away.
Felicia Koch (31), a bank employee who drove from Manchester, New Hampshire, holds up a sign saying simply: "We thank you" as she waits her turn to go in. "I'm a born-and-bred New Yorker," she tells me. "They violated my town. This is a way of dealing with my grief."