The first day - when 56 people were killed in underground tunnels and a bus - seemed quite unreal. The intended were surely not ordinary people in ordinary private places on the fringes of public life, but the City itself, or West End, or the university-land of Bloomsbury. Strategic, important, public places; picture-postcard places whose existence had no direct link to the small lives of ordinary people.

July 21 was a bit closer to home. The pattern of attacks, repeating the first, made it clear that it really was any old Tom, Dick or Harry in any old Tube stop that the killers were after - you or me.

On the Saturday after July 21, zigzagging across West London backstreets, I found strands of police tape like a spider's web all around dreary old Shepherd's Bush Tube, at the Wood Lane and Uxbridge Road end. I'd only the flimsiest of excuses to think of that ugly and unpoetical bit of town as my home territory - six months living here a long time ago - but it made all the difference. "How dare they do this HERE?", I spluttered furiously, accelerating away towards the safety of Chiswick, feeling as invaded and threatened as if I, personally, were being pursued by an army of iffy-looking youths with rucksacks full of nails.

The arrests and shootings of the subsequent days only reinforced that sense of being personally under attack.

Stockwell Tube has been a grim sort of place for as long as I can remember. It's on a roundabout flanked by tall neglected townhouses and outbreaks of 1960s concrete brutalism. Over the road one way is the Swan Irish pub. Sideways is the striped awning of Jack's Superstore, where you can get all the baked beans, sliced bread, frankfurters, chocolate, beer, coke and crisps you can take.

The Tube often has a police sign up with requests for information along the lines of "SERIOUS INCIDENT ON SATURDAY, A MAN WAS FATALLY STABBED, DID YOU SEE ANYTHING?" When I used to live there, parties in the estate over the road went on all night. Pulsating cars with space-invader lights underneath pounded up to join in. Police helicopters roared up the street before dawn.

Nowadays these memories seem quite nostalgic compared with the area's new worries. A raid on a suspect's flat just over the road from my old home; 20 police at the door yelling through a megaphone for anyone inside to come out; women led away in handcuffs; terrified small children. None of the neighbours talked much about the suspect's family. I could see why. Their fear for themselves and their own families had grown overwhelming.

The Russian writer Lev Tolstoy once described the feelings of villagers coming back to find their homes set on fire by the Russian army as "not hatred but something deeper and more primitive - the kind of contempt and instinct to destroy, exterminate, crush underfoot that you might feel for spiders or snakes or any other vermin." The birth of this feeling in London is changing the city.

It's taking a variety of shapes. It made police shoot dead a suspect at Stockwell Tube, only to discover too late he was an innocent Brazilian electrician. It makes some people quietly get off their Tube carriage if a young Asian man with a rucksack gets on. It's sent race hate crime up 500 per cent in July. It's making North London's internationalists wonder whether they should be quite so keen on open-door immigration policies. And it's making Charles Kennedy rally behind Tony Blair's proposals for tough anti-terror legislation, which he'd have choked on a few weeks ago.

On the flowers left for Jean Charles de Menezes, the dead Brazilian man, at Stockwell Tube, the condolence messages are angry. "Money does not pay the loss of an innocent life, neither makes-up clumpsy and showing-off measures!!! Justice!!! Justica!" - read one, taped to the wall next to the fruit stall. Another said: "They're killing us, brother - here not just in Iraq."

Maria da Silva, plump, short-haired and in her late 30s, a tourist from Sao Paulo, had come from Seven Sisters in North London to pay her indignant respects. "It's my first time in England and I will never come back here, never," she said grimly. She'd believed that she was coming to a happy-go-lucky, tolerant, respectful country. But now she knew - that place is vanishing fast.

Even if they can't always make their nailbombs explode, this may be where the terrorists score.

Slices of London: e-mail Urban Fox here