More than once, I asked myself - why the hell are we doing this? Putting on a convention with more than 150 speakers in eight different cities across the United Kingdom at the same time as maintaining an alliance of about 50 organisations, not all of whom loved one another - or us - with the weaving, ducking and diving that entails can be demanding.

You begin to glimpse the morbid addictions that fill the life of the seasoned political campaigner. You find yourself developing skills of appearing to agree when you don't, of smiling when irritated, of asking for money without the slightest shame, of reading and sending more emails than is recommended in a lifetime. Your language deteriorates and by degrees you morph into a version of Alastair Campbell, preoccupied by slights, losing friends fast and living off chocolate biscuits.

But you realise this is what politics or, at any rate, activism, is about - trying to have a direct impact on events and opinion, rather than just observing from the wings. It is much harder. You find yourself in the same marketplace as everyone else, desperate for attention and wondering why the broadcast media only want celebrities to explain the great crisis that threatens our free society. It has been a chastening experience to discover that we were more interesting to Russian TV than BBC Parliament, which dismissed a convention representing all parties with "our strong preference is for events based on debate between two sides of an argument".

There have been a few disasters. The worst for me involved historian David Starkey. On Thursday, Dr Starkey sent an email pulling out of the Convention on Modern Liberty because he had mistaken the time of his lecture - "The Antiquarian Endeavour" - in Stoke-on-Trent. This was irritating because, at Dr Starkey's request, Matt Brian of the CML research team had spent a couple of days doing a paper on the origins of the British police. I sent a less than flattering email to the convention office. To my mortification, I sent it to Dr Starkey by mistake. He may forgive me for this rudeness sometime in the next geological age, though I doubt it. At any rate - my apologies to Dr Starkey.

But there have been many triumphs, not the least of which was getting the convention off the ground. We started in June when David Davis resigned his seat in parliament and the Rowntree Trusts approached my co-director Anthony Barnett with a proposal to hold some sort of public meeting to discuss the state of liberty in Britain. The idea wasn't to form another civil liberties organisation, but to spark a political movement by laying out an argument and the facts of Labour's erosion of our constitutional rights.

Our ambitions grew into the cross-party event that took place yesterday, featuring some of the best minds in the country, a true convention that drew from all ages and persuasions and was united by the fear of what Labour is doing, the conviction that politics is somehow not working and that parliament is failing to guarantee the rights and liberties of the people. It was my idea to call it a convention.

Perhaps, rather grandiosely, I wanted to evoke the Philadelphia Convention of 1787, where delegates met to address problems in the government of the new United States of America and came away with the US Constitution. It is now the belief of many that the United Kingdom needs a written constitution and a Bill of Rights to protect the people from a parliament like this one.

Naturally, not many MPs agree but we were pleased to gain the crucial support of two party leaders. Before David Cameron's son died he reacted to the convention report on the loss of rights by the University College London Student Human Rights Programme. "When academics look back on Labour's time in power," he said, "the erosion of our historic liberties will surely be one of its most defining, and damning, aspects. Things we have long thought were part of the fabric of liberty in this country - such as trial by jury, habeas corpus with strict limits on the time that people can be held without charge, the protection of parliament against intrusion by the executive - have been whittled away."

Nick Clegg said: "We are the most spied-upon country in the developed world, with a million innocent people's DNA on a criminal database, more surveillance cameras than anywhere in the world, parents snooped on by council officials checking up on where children spend the night, and ceaseless attempts by government to limit our freedom of expression. That's why the work of the Convention on Modern Liberty is so important in highlighting the liberties we have lost and inspiring a new alliance in Britain to take our freedoms back."

If there was one man who kept us going through the eight months of preparation and planning, it was not Cameron or Clegg, but justice secretary Jack Straw, now carving out a historic role for himself as one of the enemies of democracy and civil liberties in the United Kingdom. Any doubts we had along the way were overwhelmed by his Coroners and Justice Bill, which contains measures that introduce secret inquests and would lift the ban on data sharing between ministries in the Data Protection Act.

In an article for the Guardian last Friday, Straw attacked the convention. "Despite the claims of a systematic erosion of liberty by those organising this weekend's Convention on Modern Liberty, my very good constituency office files show no recent correspondence relating to fears about the creation in Britain of a 'police state' or a 'surveillance society'." Failing to address Dame Stella Rimington's fears, he went on to claim that Labour "had done more than any government to extend liberties and constrain government".

And this from a man who stood up in parliament last week to veto the publication of cabinet minutes on the decisions to go to war - no doubt to protect himself - and who has tabled amendments to the Policing and Crime Bill that would give ministers the power to retain data - DNA, CCTV footage etc - for as long as they like, which , among other things, goes against the recent European Court of Human Rights' judgment about the retention of innocent people's DNA.

He is, quite simply, without shame, a disgrace to his office and parliament.

But yesterday, I hope, a movement was born and the standard raised against Straw and those who would destroy rights and liberties that have taken 800 years to accumulate.