Shi Tao is a Chinese writer, journalist and poet who, four years ago, did something that in a free country such as Britain would be regarded as routine and banal. In China it turned out to be very dangerous indeed.

Shi Tao attended an editorial meeting of a business magazine at which writers were informed of a directive from the official Chinese Propaganda Department regarding how the media were to cover (and not cover) the fifteenth anniversary of the Tiananmen Square protests. The directive was read aloud, and Shi Tao took notes. Later he posted these notes on an overseas website. For this supposed "crime" he was sentenced to 10 years' imprisonment.

The Chinese state is paranoid about what appears on the internet and employs squads of snoopers to monitor websites and to track down anyone whose contributions it regards as inappropriate or as in any way violating the country's strict controls over freedom of expression. This was how Shi Tao was trapped.

Now his cause has been taken up by the writers' organisation PEN International, which believes that his sentencing and imprisonment are in direct breach of both the International Covenant on Civil and Political Rights (to which China is a signatory) and the UN's Universal Declaration of Human Rights.

Shi Tao's case will be one of several highlighted in Glasgow next week when the Scottish centre of PEN hosts a conference that will discuss the plight of writers who are languishing in prison cells all over the world. Delegates from four continents will be attending. The sessions will be private, apart from one public session that will be aimed at informing young people about the importance of freedom of expression and the threats to it worldwide.

It is just over 80 years since, also in Glasgow, a meeting was held to found the Scottish Centre of PEN. The inspirational figure who convened the meeting was the man who was undoubtedly Scotland's greatest writer of the twentieth century, Hugh MacDiarmid. He had sought and received the support of fellow writers such as Compton Mackenzie, Neil Gunn and Helen Cruickshank, but he was the driving force in the formation of PEN Scotland.

MacDiarmid is a good example of a writer who would push dissent to the limits; he would not have lasted long in an oppressive state. In the 1920s he was campaigning for, among other things, Scottish independence, the rights of the unemployed and an end to war (though he had served in the First World War and was to contribute to the Second World War effort, helping to make gunshields in a Scotstoun engineering shop).

He was also promoting himself, in the words of his friend and biographer Alan Bold, as "the self-appointed saviour of Scottish culture in the twentieth century". The way he went about this grandiose task infuriated many people; for example, he made outspoken attacks on the much-loved poetry of Robert Burns, whose colossal reputation he denounced.

Ever the controversialist, MacDiarmid's capacity for mischief-making reached its nadir in 1956, when, following the Soviet Union's brutal invasion of Hungary, he publicly supported the Soviets - and flamboyantly renewed his membership of the Communist Party just as many writers were quitting it.

MacDiarmid's tempestuous life was full of contrarian trouble. Indeed, he was probably far more of a natural dissident that Shi Tao (who seems to be a modest and gentle writer). But troublemakers and members of the awkward squad are essential players in liberal societies, even when what they have to say seems provocative or downright silly.

When the eminent Labour politician Nye Bevan bitterly criticised Britain's conduct of the war in parliament in the early 1940s, Churchill became exasperated and condemned him as "a squalid nuisance". Yet he also conceded that Bevan's ability to utter his "diatribes" underlined the "unbridled freedom" of parliament - one of the freedoms for which Britain was ultimately fighting. Dissidents may be pests, but they are often useful and necessary pests.

Writers perhaps benefit more than others from freedom of expression but it is a basic, indivisible human right and one of the foundations of a decent society. For obvious reasons, repression in China is much in the news at present, but writers are censored, imprisoned and tortured in many other countries.

The Glasgow PEN conference might not be able to achieve much in the way of practical aid for them, but campaigning and profile-raising can play their part. As Fiona Graham, a Scottish committee member of PEN and one of the conference organisers, says: "Silence is one of the most powerful weapons of oppression. It is the responsibility of PEN International and its Writers in Prison conference to break that silence of oppression and to raise people's awareness."