Normal life resumes. But this is a city on edge, full of frazzled nerves, tension and drawn faces

New York

Humans in wartime behave incredibly, and so it is in New York. Three days after the unspeakable occurred at the bottom end of Manhattan, residents of the city were trying to be normal. Catching buses, taking children to school, returning to work. But their faces told a different story.

Humans in wartime behave incredibly, and so it is in New York. Three days after the unspeakable occurred at the bottom end of Manhattan, residents of the city were trying to be normal. Catching buses, taking children to school, returning to work. But their faces told a different story.

The city is very much on edge. It is wound tight and almost anything triggers a return of fear. The bomb scares are the worst. They happen all over. On Thursday, an abandoned suitcase near where I am staying had police scrambling. Evacuations were ordered throughout the day in locations all over city, from Grand Central Station to Macy's Department Store – even the studios at CNN.

People here are pretty tough and good-humoured. But they are tired now. When the evacuation was ordered at the train station, I watched women running down 42nd Street wailing in panic, afraid for their lives. Workers in adjacent office blocks who had come out for their lunch , bolted back inside. It was too frightening to be out.

Nerves are frazzled. I paused for a quick dinner at a Ukrainian diner on Wednesday evening with some friends. We talked about what happened. That is all anyone in the city is talking about. Of course it is. It is strange to be in a place where everyone has the same thing on their minds. We are all still trying to understand. To believe what we have seen.

Suddenly, police cruisers, several of them, skidded to a halt outside. Everyone emptied out from the café and from everywhere else around. What now? There had been a shooting in the restaurant two doors away. For the past few years, New York has been different from how it used to be, when people were afraid to go out after dark because of gun-toting drug dealers. It had gained a new self-confidence and sense of security. That has been wrecked. Tempers are being lost. The shooting, thank goodness, turns out to be a false alarm and the police go back to other business. But, just minutes later, two men brawled on the pavement, yelling. Everyone needs to calm down.

There has been some tranquility in parts of the city, though of an eerie, post-traumatic kind. On Thursday night, I returned to Avenue A to visit, for a second time, a mural of the burning towers that has become one of several impromptu shrines in the city to those who have perished. I ate at a restaurant that had chairs and tables set outside, nearby. This was south of 14th Street, just inside the zone that had been sealed off to all traffic by the Mayor. The avenue had become a promenade. Everyone was out, walking, talking sharing their stories. The army was manning the barriers to stop the cars. It felt a bit like a town under martial law.

Everywhere, people had put American flags out. On fire escapes mostly or draped from the edges of their flat-roofed buildings. The strange peace was shattered by someone above, turning hi-fi speakers onto the street and playing, at ear-piercing volume, a hard-rock version of America's national anthem. It was an ugly, screeching version and several police officers rushed up the apartment's stairs to shut it down. But it is patriotic, nonetheless. Those who had been walking outside stopped to listen. When the music halted, they broke into applause.

The Stars and Stripes flags continue to spread. I noticed the phenomenon first on the morning after the horror, when I visited the site of the towers. Rescue trucks, covered in ash and debris, had small American flags fixed to their wing mirrors. Now they are all over. The lift in the block where I am staying has one – it appeared yesterday morning, drawn by a child, evidently, in red and blue crayon, the 50 stars painstakingly drawn in pencil. "They are gone, but they are not forgotten", it says. One of the people I am staying with is writing a poem to the dead. She will take it to Union Square.

That is where the biggest of the shrines has popped up. I am reminded of all the Diana shrines when she died. There are lots of flowers and candles. Someone has made a 5ft sculpture in the shape of a peace candle. Over the past three days, scores of people have written messages and prayers on sheets of brown paper taped to the stone of Union Square park. Yesterday, they were all reduced to sad, poignant mush by a night of hard rain. New sheets will appear when the rain stops. Including my friend's poem.

So many of the messages contain a warning for fellow New Yorkers: the temptation will be for us to turn on fellow New Yorkers who happen to be of Arab descent. Don't let that happen, they say. But it is starting to. Yesterday, a man asked me where I had bought my newspapers. I pointed to a deli and newsagent on 14th Street. "At the Arab store?" he snarled at me, suggesting I had committed a sin. I have a Moroccan friend here who works for a communications company. He didn't go out for two days and was afraid to go to work today.

There are also shrines at all of the city's fire stations. There is one just near me. It is a fire tender, Engine 5, and everyone inside was sent to the financial district when calamity struck on Tuesday. I don't know how many of the station's brothers were lost, if any. I am not about to go in and ask, but more than 300 have vanished from the service citywide. Everyone knows the pain they must be feeling inside and so flowers, candles and cards are piling up outside its doors on the pavement. "For all your fallen fellow firefighters ... Thanks," one card reads. "To our Friends at Engine 5 (Our Heroes)", says another. The firemen have put out a large card of their own for us to read, "The New York City Fire Department Thanks You For your Sympathy and Support". Written messages have proliferated across the city. Also in my lift are computer print-outs with information about all the practicalities of getting around the city. Someone thought to put these in there and update them a couple of times a day. It tells about the parking rules – all normal laws, like the need to feed the meters, are still suspended – which parts of the city are sealed off and which are accessible, which airports are open (none) and which trains are running (a few).

Then, of course, there are the flyers posted by the relatives and friends of those still missing. Most of them are concentrated around the hospitals where the injured were treated – there are none left now to treat, tragically – like Bellevue on the East Side or at the Armoury, a massive brick hall that has been commandeered as a reception point and counselling centre for all relatives. The walls of that block, the post boxes, the telegraph polls are festooned with flyers. People hand them out and have them sellotaped to their backs. Telephone shelters used to have flyers advertising call boys and prostitutes, now they carry flyers. Death has replaced the sex industry.

The flyers constitute shrines of their own, in a sense. Ostensibly, they are put up by people who still hold some hope that someone, somewhere will have news of their missing loved one. But most know, in their heads if not their hearts, that they are vanished for good, beneath a ghastly tomb of pulverized cement and glass. Thus, they are almost like temporary gravestones. RIP sheets printed at copy shops. Everyone in those pictures looks so handsome or beautiful or young. It is natural that they should – it is the best pictures of the missing that are going to be chosen, not the indifferent ones.

The rain brings mixed feelings. It makes the rescue effort even more difficult, of course. The terrorists chose a brilliant day for their assault, presumably on purpose, and the skies remained brilliantly clear, until about midnight on Thursday. The fine spell was broken by a huge clap of thunder in the early hours of yesterday. It woke me and I sat up in bed at once, scared. I thought I had heard another building crashing down. People asleep across Manhattan must have done the same thing. We had all been made nervous again by evening news reports that the authorities had arrested more people at the New York airports with fake pilot's ID and maybe knives. Could it happen all over again? Will the Empire State Building be next or the UN building? People in the city are frightened not just because of what has happened but of what may happen next.

The wet weather now provides a kind of cleansing, both real and figurative. The smoke was mostly gone by yesterday morning. For more than two days, it had billowed out from the disaster zone as new fires began. The smoke gave Manhattan the air of a war zone, or a battlefield just after an attack. Sometimes, the smoke would blow north. Even up in midtown, perhaps five miles away, we could smell it, the burning. Some in the street donned masks. The smoke wasn't that bad, but perhaps they were wise. The environmental agencies have repeatedly reassured us that, while asbestos was clearly present in the twin towers, there were insignificant amounts of it floating in the air. Many residents were sceptical.

You can't buy flags in New York any more. Dealers say there hasn't been a rush on Old Glory like this since 1991 when the Gulf War broke out. One shopkeeper, Barry Kessler, an owner of the shop, Five Boro Flags, says he has one left, a giant of a flag, 20ft by 38ft. But he intends to donate that to the city, in the hope that it might be draped over the ruins at the World Trade Centre. "We want to get this flag on what remains of that icon", he said yesterday. "We want there to be something nicer for the cameras to show".

Everywhere, there are examples of New Yorkers pitching in. The easiest way, of course, is to give blood. Thousands and thousands have done it. I can't because I come from the United Kingdom, incubator to "mad cow disease". Even the city's funeral homes have come together to offer burials to all city workers who have perished – firefighters and police officer – for free. Every one of these heroes will be given coffins, for example, worth more than $2,000 (£1,360). No city on earth is more mercantile than this one but this is not a time for profiteering.

Getting back to normal is not something that will happen in this city for a very long time. Getting back to something like a semblance of normality is already happening though. Commuters have returned and children are attending classes. They raised the curtain at The Producers on Broadway, the smash-hit Mel Brooks musical, for the first time again on Thursday night and, amazingly, it was a sell-out.

A producer took the stage at the start to deliver a few words. "As we say in theatre," he said, "the show must go on".