For 30 years I have been travelling to unfree places, from East Germany to Burma, and writing about them in the belief that I was coming from one of the freest countries in the world. I wanted people in those places to enjoy more of what we had. In the last few years, I have woken up - late in the day, but better late than never - to the way in which individual liberty, privacy and human rights have been sliced away in Britain, like salami, under New Labour governments that profess to find in liberty the central theme of British history.

"Oh, these powers will almost never be used," they say every time. "Ordinary people have nothing to fear. It affects just 0.1%." But a hundred times 0.1% is 10%. The East Germans are now more free than we are, at least in terms of law and administrative practice in such areas as surveillance and data collection. Thirty years ago, they had the Stasi. Today, Britain has such broadly drawn and elastic surveillance laws that Poole borough council could exploit them to spend two weeks spying on a family wrongly accused of lying on a school application form. The official spies reportedly made copious notes on the movements of the mother and her three children, whom they referred to as "targets", and watched the family go home at night to establish where they were sleeping. And this is supposed to be modern Britain?

Let's be clear: though the Stasi headline is irresistible, such Stasi-nark methods do not yet make a Stasi state. The political context is very different. We don't live in a one-party dictatorship. But nor is this just "an isolated case", as ministers always protest. Almost every week brings some new revelation of the way in which our government has taken a further small slice of our liberty, always in the name of another real or alleged good: national security, safety from crime, community cohesion, efficiency (ha ha), or our "special relationship" with the United States.

Liberty comes last. As Dominic Raab writes in his excellent book The Assault on Liberty, this government "has hyperactively produced more Home Office legislation than all the other governments in our history combined, accumulating a vast arsenal of new legal powers and creating more than three thousand additional criminal offences". At a press conference today, the organisers of next week's Convention on Modern Liberty - whose moving spirits include the columnist Henry Porter and the democratic activist Anthony Barnett - will present a first attempt to catalogue the liberties we have lost, in a list compiled by the University College London Student Human Rights Programme.

Other free countries, including the US, have overreacted to the threat of terrorism, violating their own basic constitutional principles and legal standards. The peculiarity of Britain is that we have nibbled away individual liberty on so many different fronts. We have been complicit in American-led torture of our own people; at the same time we have eroded free speech in ways unthinkable in the US; and we have become what Privacy International calls "an endemic surveillance society".

Yes, fighting terrorism requires some restrictions. Yes, you can make a crime reduction case for some CCTV. But we have more CCTV, a larger DNA database and a more ambitious (and unworkable) National Identity Register scheme, as well as more police powers and more email snooping than any comparable liberal democracy. On top of which we have a bureaucracy so centralised and incompetent in managing this mass of data that it lost two computer discs containing the child benefit details of 25 million people.

What's more, the certain loss of liberty will often not result in the alleged gain in security or efficiency. So, for example, Gordon Brown and his ministers went on pressing for 42 days' detention without trial, despite the fact that two former heads of the country's security service, the director of public prosecutions, the former lord chancellor, attorney general and lord chief justice - in short, almost everyone in a position to know - said it was wrong, unnecessary and counterproductive. How can a government of intelligent and often liberal-minded persons behave so illiberally, arrogantly and stupidly? What screw have they got loose? What nerve is missing?

Fortunately, the fightback has begun. It has been led by three groups: judges and lawyers (witness the excellent article by the former senior law lord, Thomas Bingham, on these pages on Tuesday); unelected peers (witness, most recently, an outstanding House of Lords report on surveillance); and a rainbow coalition of journalists, academics, writers, artists, thinktankers, civil society activists and citizens, of left and right, young and old, some of whom have now joined together to launch the Convention on Modern Liberty.

Notably absent from this list is the one group who should be on the frontline when it comes to the defence of British liberties: our elected representatives. This is not just a New Labour failing. With a few notable exceptions, such as David Davis, most of our MPs have been complaisant and pusillanimous beyond belief as our liberties have been chipped away. So, for example, last week the home secretary pathetically and idiotically banned the Dutch MP Geert Wilders from entering the UK to show his noxious and offensive anti-Islam film at the invitation of members of the House of Lords. Result: a curtailment of free speech that gives Wilders more free publicity than he could otherwise have dreamed of. And how does the Liberal Democrat home affairs spokesman Chris Huhne react? Oh, that's all right, he says, because the film is really offensive. Well, d'oh. Call yourself a liberal? John Stuart Mill would be turning in his grave. And I shall need some convincing that the Conservative frontbench are going to be any better.

I'm not sure I fully understand all the reasons for this cravenness, but here's one. A couple of years ago I asked a very senior New Labour politician if his government had not got the balance between security and liberty wrong. "Well", he replied, "one thing I can tell you is that if you ask the British people they will always choose more security." And this is where the ball comes back to us. Since our leaders are now mainly followers - following the latest opinion poll, focus group or newspaper campaign - it's up to us, the people, to change their view of what "the people" want.

That's why it's so important for as many of us as possible to turn out across the country for next week's Convention on Modern Liberty, to send an unmistakable message to the government and parliament of a country which is still a long way from being Burma or the old East Germany. And then we need to go on lobbying our MPs, in every manner known to man, woman and child, until that message penetrates their thick and supine skulls.

To be honest, I still cannot quite believe this is happening to my country. It feels like a bad dream. But it is happening, and we must stop it. Now.

Timothy Garton Ash is among the speakers at the Convention on Modern Liberty, which takes place in London on Saturday 28 February, with other sessions in Bristol, Birmingham, Manchester, Cambridge, Glasgow, Belfast and Cardiff. For more information and to buy tickets, see modernliberty.net.

www.timothygartonash.com

• This article was amended on Thursday 19 February 2009. We incorrectly referred to the University College London Student Human Rights Programme as the University College Law School Human Rights Programme. This has been corrected.