This is the statement of principle that Saturday's freedom of expression rally has been called to defend. How can anyone disagree with these progressive values? I can't, and that is why I will be joining the thousands in Trafalgar Square.

The rally is backed mostly by secular, humanist and libertarian groups, but with support from some left-wingers and liberal Muslims.

Some of my friends on the left are refusing to take part. Preferring to remain marginal but pure, they object to the involvement of right-wing groups like the Libertarian Alliance and the Freedom Association. I share their distaste for these groups. But my participation on Saturday is based on supporting the statement of principle, not on who else is taking part. I will not let the dubious politics of others dissuade me from supporting what are important, progressive humanitarian values.

Sections of the left moan that the rally is being supported the right. Well, if these socialists object so strongly why don't they organise their own demo in support of free speech?

The truth is that is that some of the left would rarely, if ever, rally to defend freedom of expression because they don't wholeheartedly believe in it. Mired in the immoral morass of cultural relativism, they no longer endorse Enlightenment values and universal human rights. Their support for free speech is now qualified by so many ifs and buts. When push comes to shove, it is more or less worthless.

As a left-wing Green, committed to human rights and social justice, I do not share the politics of some other speakers and rallyists. But this is the whole point of Saturdays' demo – to defend the free speech of those with whom we disagree.

While I support the right of newspapers to publish cartoons satirising any religious or atheist leader, there are bigger, more important free speech issues to fight.

When I speak in Trafalgar Square on Saturday, I will defend Muslim communities against prejudice and discrimination, attack the BNP and the war on terror, and condemn the government's erosion of civil liberties and individual freedom.

My speech will also assert the right to condemn British troops in Iraq and Afghanistan, urge less state secrecy and more freedom of information, and call for the disestablishment of the Church of England and the freedom to insult the Queen, Prime Minister and Archbishop of Canterbury.

When it comes to free speech, I am an equal opportunities free speecher. I even defend the right of others to mock and ridicule me. I may not like it. It might be unfair. But that's democracy.

Some critics are mischievously portraying Saturday's protest as an anti-Muslim rally. I condemn any attempt to demonise or scapegoat my Muslim brothers and sisters. I also reject the suggestion of a clash of civilisations.

Both fundamentalists and progressives can be found in all faiths, politics, ethnicities and cultures. No society has a monopoly of enlightenment and plurality. Muslim societies like Bangladesh have produced Enlightenment icons like the feminist writer Taslima Nasreen; while supposedly cultured nations like Britain and France have spawned the Dark Ages ignorance of the British National Party and the Front National.

When considering the vexed question of the limits to free speech, perhaps we should start with first principles:

Article 19 of the UN Universal Declaration of Human Rights states that: 'Everyone has the right to freedom of opinion and expression.'

By this standard, freedom of expression is a fundamental human right for every person on this planet. It is a right for all, not some. If we expect free speech for ourselves, then we are duty bound to ensure that it also exists for others.

Contrary to what the cultural relativists try to suggest, freedom of expression is not a western value; it is a universal humanitarian value that every member state of the United Nations has pledged to uphold. By demanding the right to free speech, we are not seeking to impose western values on non-western nations. We are merely asking the governments of the world to honour the human rights commitments they agreed when they signed up to the UN.

Free speech is one of the litmus tests of a free and democratic society. Alas, not everyone shares a commitment to democracy. To maintain their power, political and religious tyrants have always censored ideas and opinions. Some liberals and left-wingers, often with the honourable motive of tackling prejudice, have also attempted to place constraints on what can be publicly said on issues such as race and sexuality. This authoritarianism lite has its downside too. Suppressing intolerant ideas doesn't make them go away. They just go underground and fester. This is not a solution.

While many people of faith have been recently up-in-arms over cartoons, plays and operas they find offensive, ironically it is the free expression they oppose that is the precondition for genuine political and religious freedom. It is in the interests of people of all political and religious beliefs - and of none - to defend freedom of expression. By defending the freedom of others we are also defending our own freedom.

The right to free speech is the surest guarantor of religious freedom. Without freedom of expression, religious minorities tend to be persecuted by religious majorities. Witness, in theocratic Iran, the victimisation of Sunni Muslims by Shia Muslims.

A democratic secular state is the true protector of all religions. It guarantees religious freedom and equality, ensuring that no one faith lords it over others. That is why, among other things, I favour the disestablishment of the Church of England, to end the privileged constitutional and legal status of this increasingly diminished protestant sect.

Freedom of expression should not, of course, be abused. A harmonious, good natured society is one where people are civil and courteous to each other. Prejudice and discrimination have no place in civilised discourse. Offensive language - whether sexist, anti-gay or racist – is rude and divisive and should always be challenged.

Those who justify legal limits to free speech need to answer a number of questions:

When it comes to censorship and bans, where do you start and where do you stop? Who decides what is sufficiently offensive to merit restriction? At what point do you draw the line? Isn't this an inevitably subjective judgement? When does a well-meaning desire to protect vulnerable communities spill over into the dangerous territory of giving some communities privileged protection and immunity from criticism?

All human beings are worthy of respect, but not all ideas deserve respect. There is, for example, no obligation to respect Nazism, misogyny, white supremacism, homophobia or creationism.

I grew up in Australia in the 1960s, during a period of McCarthyite-style red-baiting. Because I opposed the US and Australian war against Vietnam, I was denounced as a communist and nearly lost my job. From firsthand experience, I know freedom of expression is a precious freedom that must be safeguarded.

That is why I argue the right to free speech can be legitimately restricted only when it involves incitement to violence or libel/defamation. The threat of violence and the spreading of untruths diminish free, honest and open debate. Otherwise, speech must remain free. The rare exceptions are instances like not being free to publish terrorist bomb-making instructions.

The price of living in a free society is that we are sometimes confronted with views we find offensive and insulting. Faced with bigoted, intolerant opinions, the most effective way to challenge them is by calm, reasoned debate to dispel ignorance and prejudice – not by bans and censorship. Physical threats and violence are unacceptable.

In January, I challenged Sir Iqbal Sacranie of the Muslim Council of Britain when he denounced homosexuality as immoral, harmful and diseased. But I did not seek to ban him, nor did I support calls for his prosecution. I defended Sir Iqbal's right to free speech. Will he and his fellow MCB leaders now defend my right to freedom of expression? Or is Sir Iqbal another of those selective free speech proponents? Freedom of expression for me, but not for you?