A friend was driving home one night a few weeks ago when she saw a flashing blue light behind her. She pulled to the side of the road and a police officer approached.

A friend was driving home one night a few weeks ago when she saw a flashing blue light behind her. She pulled to the side of the road and a police officer approached. "Good evening," he said calling her by her name. The hairs stood on the back of her neck.

She asked: "How do you know my name?" He said he had checked her number plate from his car. Then he said: "Step into my vehicle please." She said: "No, thank you." He repeated it and looked surprised when she refused again. The exchange interested her because clearly he was unaccustomed to having his invitation/instruction refused. "It's just so cold outside," he said.

He told her very civilly that she had failed to see a poorly signalled lane closure, which many other motorists had also missed. Within minutes she was on her way home but more rattled than the officer could imagine. You see, my friend grew up in Africa where she learned from bitter experience that a law-abiding citizen can find themselves on the wrong side of police power. It has made her vigilant of civil liberties.

I can imagine her reaction to the police raid on Damian Green, the shadow immigration minister. She'll be appalled, as I should be. Instead I'm delighted. I'll explain why later.

Like my friend, I have history which makes me wary of abuse of power by the police. I grew up in Northern Ireland, where officers were almost exclusively drawn from a community that wasn't mine.

Police stations were sandbagged and policemen always carried guns. When I was in my teens it dawned on me that the police were not necessarily on my side, even though I was a law-abiding citizen from a law-abiding family.

As a student I joined the civil rights movement and (in pursuit of one-man, one-vote in local authority elections) was on a march from Belfast to Derry which was ambushed at a place called Burntollet Bridge. We were pelted with stones from a hillside on our right and broke to the left across a field and into a river. It was January.

Some people had their heads cracked open. I just got a soaking before I was rescued by a farmer who had come to stop our progress and changed his mind when he saw the violence. He drove me and another marcher to hospital in Derry.

An elderly local woman then took me in, gave me tea and loaned me one of her dresses. A friend found me and was leading me through winding streets towards the hotel where the students were encamped when, approaching round a corner and blocking the entire street, came a water cannon with men in riot gear on either side of it. The water would have knocked you off your feet.

I turned to my companion and said: "Ring the police." He said: "Those are the police." My point is that you can find yourself on the wrong side of the police, without ever breaking the law.

It is, therefore, important that the police force is a measured and impartial organisation and that it is not given too much unchecked power. It is important for it not to use water cannon, a sledge-hammer - or a bunch of anti-terrorist officers - to crack a nut.

So I am delighted by Damian Green's arrest. If it takes his interrogation, the raid on his family home by anti-terrorist police, his teenage daughter being reduced to tears, to make everyone sit up and take notice of what is happening to our civil liberties, then at least a public service will have come from this.

The police rifled through private letters and confiscated bank statements. They even tried to take a computer belonging to his wife, a lawyer. Rightly, she protested. The icing on the cake was when they marched into the mother of parliaments and raided Damian Green's office. Once again, they took computers, no doubt containing private correspondence from constituents.

There has been a very public cry of outrage. There have been accusations of breach of parliamentary privilege (though an expert says this extends only to what MPs say in the chamber). The Speaker of the House of Commons may be looking at the end of his career because he didn't protest against the raid. The Home Secretary has fallen under suspicion of knowing more than she will admit to in advance of the action. The Prime Minister is being held up as an admitted receiver of leaks when he was in opposition.

That police action is visibly and demonstrably wildly disproportionate to any crime committed or any threat to the state, could not be plainer. They have dug a pit for themselves which is hopefully large enough to curb their recent draconian tendencies.

I am not being malicious about Damian Green and his family when I cheer the fact and the manner of his arrest. I am grateful to them because if similar tactics were used against some low-profile Joe Citizen, we would never have heard about it. Or if we did, both press and electorate would think the police knew more than they were making public. If the man arrested had been called not Joe but Hussein or Ahmed, we could have been blinded by smoke with no fire at all.

And, unlike Damian Green, they might have disappeared into custody for 28 days.

Journalist Sally Murrer was subject to similar tactics and was charged with "aiding and abetting, counselling or procuring misconduct in a public office" (the offence for which Damian Green is being investigated). Her telephone calls were bugged, her home and office were raided. She was accused of receiving from a police officer the sort of information many would tell you over a pint.

The case against her was demolished, just as the Green case was hitting the headlines. The court ruled that leaks to journalists are not criminal unless they involve matters of national security or impair the investigation of serious crime. Quite.

I can see that (just like the private citizen) it is reasonable for the police and for the Home Secretary to expect discretion in their inner sanctums. I am surprised that a former (maybe even current) party political activist, like 26-year-old Christopher Galley, was cleared to work in Jacqui Smith's private office. Leaking for political ends is a serious matter since the political neutrality of the Civil Service is integral to our democracy. But it is an organisational and disciplinary matter for Jacqui Smith and her mandarins to deal with.

Of much greater importance is the newly muscular attitude of both government and police. Because of the threat of terrorism, the police have new powers to protect us. They and we must be vigilant lest they are used to oppress us.

The government has had a reminder that people of all political persuasions in this country put one shared value above party loyalties - the value is personal freedom.

Thanks to their choice of Damian Green, the Metropolitan Police has alerted us to how far that freedom is already threatened. For that, we have cause to be grateful.