THE WAILING grew louder. At first I thought it was a siren - an ambulance or a police car, unremarkable on the city streets. But the fearful faces of everyone else made me realise the sound was human. On the back seat of my car my dog crouched, ears flattened. The wailing was loud and seemed to be everywhere.

Getting out of my car, I saw where the noise was coming from. On the far side of the street a woman lay curled under a billboard. The continuous noise she made seemed impossible. A thick tangle of dark hair obscured her face. She held her fingers tight to her temples, pressing, pressing, as if trying to relieve the pain.

I looked around me. People were staring from the top of a double-decker bus. People in clusters in the street were looking and whispering. A nurse appeared from a doctor's surgery, watched and then went back indoors again. And still the unnatural screaming noise went on.

I crossed the road and went up to the woman. 'Do you need any help?'

The screaming stopped immediately. She looked at me, eyes full of suspicion.

Then she got up and weaved her way down the pavement. Behind me I could hear someone shout: 'She kicked a baby.'

My car was badly parked and I kept hoping for someone in authority to come and tell me off. 'She's mad, love, don't worry, she should be locked up,' said an older woman.

'I know,' I said. 'We have to phone someone. She shouldn't be on the streets.'

'Closed everywhere down, haven't they?,' sighed the woman. 'That's the bloody trouble, this bloody government.'

I went up to the group of people clustered outside a fried chicken takeaway. A group of women stood protectively around a little girl of five or six, who was sipping a drink, wide-eyed and subdued.

'Just came up and kicked her in the back.'

' Kicked her out of the blue.'

'I gave her a good kick, got her in the back. What does she expect?'

'If it was my child she kicked, I'd kill her.'

Me too. If she attacked my child I'd want to kill her.

'Look, has anyone phoned the police?' I asked. 'She shouldn't be on the streets.'

'No point, love. The police don't do nothing.'

'I know her, she's always going for children.'

As we watched, an old man wearing a suit and cap approached. The woman swung viciously and kicked him between the legs. He doubled up, then stood up and carried on along the street. He asked for no help, and nobody offered any. He carried on about his business. The woman disappeared from view.

I got back into my car and pulled out into the traffic. I didn't know what to do. This woman was obviously not only mentally ill but dangerous. I caught sight of her and drove along the road slowly, keeping level with her, irritating all the other drivers. I indicated left so they could overtake me.

I have a car phone and I phoned the police. Even if they could do nothing, I felt, they still needed to be told. I didn't want to read about some child pushed under a bus by a madwoman a week later. I was put on hold.

The woman staggered along, drifting from side to side, taking up the whole pavement. She was tall and looked strong. Her clothes were a mishmash. Purple bomber jacket, man's trousers and - incongruously - bright-blue high-heeled ballroom dancing shoes. Tears streaked her muddy face as she rubbed her eyes, sobbing.

I explained what had happened to the police on the phone. I said where I was. They said they would contact a patrol car and then put me on hold again. Behind me a lorry had difficulty finding space to overtake.

Out of nowhere another woman appeared; furiously, she attacked the madwoman. She dragged her to the ground and kicked and thumped her with a frenzy of blows. I wound down my window and screamed out at her to stop: 'You can't do that. I phoned the police.' How ineffectual I sounded. 'Speak to me,' I pleaded with the telephone. 'You've got to come right away.' The woman curled her body protectively, arms wrapped around her head.

People watched but no one stopped, no one helped. Then she stood up, pushed her assailant away and walked on. Sobbing visibly, she reached in her pocket for a purse. Spying on her desperate misery was very painful. She crossed a road, shouting loudly at a car that got in her way.

I followed. My left foot was shaking so hard I couldn't use the clutch. When she went into a shop I pulled into the side of the road and put the car in neutral. She came out of the shop carrying a can of beer.

A young woman in a denim jacket came walking towards her. She had long clean blond hair and a net shopping bag. As she drew level the madwoman kicked her savagely in the back.

The young woman shied away and carried on walking. She didn't look for help, nor was she offered any. The madwoman went into a hotel.

I parked outside and phoned the police again. Ten minutes later they appeared, a man and a woman. A truncheon poked out of the WPC's pocket. I identified myself and described what I had seen.

'You see, what you're saying to me is basically a crime without a victim,' explained the helpful policeman. 'Unless a victim comes forward, there's not a lot we can do. And if they've beaten the shit out of her, no one's gonna come forward.' He had a point.

'I think I know her. A Whittington absco,' put in the WPC. She got on her radio. 'An absco, that means an absconder. She discharged herself from the Whittington Hospital. We can take her back and they'll keep her for a couple of days. But don't be surprised if you're driving past here next week and you see her on the street.'

This is my city. This is where I live. I don't want to read about a woman, so obviously in need of help, harming herself or anyone else, in my newspaper. I don't want to see vigilante mums participating in a circle of assault.

'I mean, I could go in there, have a fight with her and get her in a van,' said the policeman doubtfully.

'Be gentle,' I said. 'She's covered in bruises already.'

The Zito Trust, offering support where community care fails, will be launched later this year.