Last weekend our nation crossed a line. On Saturday, a visiting tourist was arrested in Melbourne by the Australian Federal Police, on instructions from the Department of Immigration and ASIO.

His visa has been cancelled and he is to be deported on security grounds.

Who is this threat to our security?

He is not a person with any history of violence. He is not a person who has been known to associate with terrorists. He has not advocated violence against anyone.

Scott Parkin is a peace activist from the United States.

While in Australia on a holiday he has participated in workshops against the Iraq war. He performed some anti-war street theatre outside the Halliburton office when he was in Sydney. Here in Melbourne, he was on his way to conduct a publicly advertised workshop on the Iraq war when he was picked up by our nation's finest.

So subversive were his activities in promoting non-violent action against the Iraq war that he was taken straight to jail (where he is being held at his own expense) and from where he will be taken out of the country.

The only reason given by the authorities is that he represents a security risk. Attorney-General Philip Ruddock confirmed this after Parkin was arrested. There are no breaches of any conditions of his visa. The authorities will not specify the nature of the security threat, and it is possible they know something that definitely justifies action of this nature. But they won't say. The only thing anyone (including Parkin) knows that could constitute that threat is that he has actively - and non-violently - campaigned against the invasion of Iraq.

There is one other thing. In the United States he was charged with a misdemeanour (a minor charge) as a result of participation in a non-violent Greenpeace protest action. If that amounts to a security threat, the Australian authorities should have been well-apprised of it before he entered the country.

It appears that Parkin is being held in jail and deported for being a peace activist. The Government has done nothing to dispel this view. One might expect this sort of thing of Saddam Hussein's regime, but not of a country with democratic principles like our own.

It is our tolerance of a wide range of views that gives our nation so much of its strength. Our ability to hear and open our minds to even radical views makes us stronger - not weaker.

But Parkin's views could scarcely be described as radical. A sizeable proportion of Australia's population - and a sizeable proportion of the US population, for that matter - share his opposition to the Iraq war. Under the present expansive and ill-defined terms "terrorist" and "security threat", ordinary Australians organising or participating in rallies, protests or public meetings could potentially be investigated by ASIO. Can it be that the expression of anti-war views threatens our security so much that such views must be silenced?

The terror laws passed by the Federal Government permit exactly this kind of action by the authorities. They give wide power to ASIO and the AFP to target individuals who engage in industrial and protest action directed towards social change.

It now appears - and the Government has not done anything to dispel this view - that the powers are being exercised against Parkin because he has expressed political views at odds with those of the government of the day. We should not merely be alert about that - we should be alarmed.

Where powers are conferred, there is always the prospect that such powers will be abused. One way to circumscribe the prospect of such abuse is to adopt a charter that guarantees internationally recognised human rights, including freedom of speech. Every other Western nation has done so.

The political process is a non-violent way to resolve conflicting values and interests. By valuing free speech and political participation, civilised societies avoid resort to violent means to resolve conflict. Erosion of our ability to participate in political life exposes us to the risk of violence.

When we violate human rights - such as the right to free speech - we compromise our integrity as a nation, and the very basis of the democracy upon which our national security is founded.

And now our terror laws, and ASIO (which has morphed into a kind of secret police) are being used against those who oppose the policies of the government of the day.

Liberal democracy is built on human rights, including the freedom to vigorously exchange ideas.

Unless Parkin's treatment is a grotesque error, we have stepped over the line separating liberal democracy from more sinister forms of government.