The ultimate impact of the World Trade Center attack on New York City - its people, finances and psyche - was incalculable in the hours following the terrorist strike.

But there was no doubt it was the deadliest day in the history of the city, which despite its reputation as a violent place had never before known war in the modern era.

Rubble inches thick covered lower Manhattan and smoke shrouded the financial district - leaving only the Statue of Liberty in New York harbour clearly visible.

"I sense a horrendous loss of life," a shaken Mayor Rudolph Giuliani said. Later in the day, he added: "It is almost impossible to describe the level of anger you have."

The attacks hit New York at one of its most sensitive points. The World Trade Center and nearby office complexes in lower Manhattan are a city within the city, honeycombed with underground train stations and shopping malls.

Some 50,000 people worked in the World Trade Center and 150,000 more passed through the complex every day.

After the attacks, the financial markets that drive the local economy were closed. A primary election to pick the Democratic and Republican party candidates vying to succeed Mr Guiliani was suspended.

"It made no sense to have an election," Mr Giuliani said. "We will find another day."

The initial response of city officials was to clear lower Manhattan so police, firefighters and ambulances could attend the victims. Police were dispatched to the New York Stock Exchange to walk some 3,000 people inside to safety.

"Walk north - if you are in lower Manhattan - walk north," Mr Giuliani told New Yorkers tuned in to their televisions. "Try to get out of lower Manhattan."

After the first two explosions, sidewalks around the World Trade Center were covered in a thick layer of dust, building cladding, glass and discarded high-heeled shoes.

Burning sheets of paper fell on Broadway's "canyon of heroes" like a macabre tickertape parade. Several blocks away, shop windows shattered. Bewildered onlookers attempted in vain to get signals on their mobile telephones.

Mr Giuliani said he went to a nearby office building after the first aircraft hit the complex. The mayor was among those who fled as the buildings collapsed - and he praised his fellow New Yorkers for their co-operation in the face of adversity.

"They were orderly. They were calm," he said. "They handled themselves better than anyone could have expected."

All hospitals in the city and in surrounding areas of New York, New Jersey and Connecticut were pressed into service.

Governor George Pataki had put the National Guard on alert so it could help to relieve city police officers and firefighters later in the day. "I have friends in those towers," Mr Pataki said. "We will be there to help."

A limited train and bus service was available out of Manhattan, but for the most part, train, bus, car and ferry traffic to the island stopped. The collapse of the World Trade Center also interrupted power on some subway lines, trapping passengers in tunnels.

To the north, in mid-town Manhattan, traffic proceeded normally. In the other boroughs of New York - across the water from Manhattan - there was also little evidence of panic.