However indignant you felt about him, and the calamitous mess over which he presides, it would be impossible ever to throw a shoe at a man such as Abhisit Vejjajiva. Among his peers, the new Prime Minister of Thailand challenges even Barack Obama for the title of World's Most Decent Leader.

As a young politician, he was a heart-throb among middle-aged Bangkok matrons. At Eton, where he was known by the name “Mark Vejj”, he was best friends with Boris Johnson. He is handsome, youthful, brilliant, cosmopolitan, impeccably well mannered and rather posh. So when he gives a speech at his old university, Oxford, tomorrow, it is safe to assume that the audience at St John's College will be keeping its brogues securely laced.

But Mr Abhisit's charm should not be a distraction from ugly truths about what is happening in Thailand. In the past four years, it has gone from being one of the most free and stable countries of South-East Asia to one of its most chaotic and divided. Writers, academics and journalists have been imprisoned or hounded into exile for harmless comment on Thailand's monarchy. Helpless boat people have been chased out to sea to their deaths. Democratically elected governments have been forced out, first by the army and then by the power of the mob.

All of this has been done with the approval - sometimes passive, sometimes explicit - of the nice Mr Abhisit. The title of his talk at St John's tomorrow, “Taking on the Challenges of Democracy”, could not be more appropriate, for Thailand's leader is indeed democratically challenged. Rarely since the days of Dr Faustus has a gifted and promising man achieved power through such grubby and disreputable means.

Since Mr Abhisit became the leader of the Democrat Party in 2005, there have been two general elections in Thailand. He boycotted the first one in 2006, which was won, for the third time in a row, by the man at the centre of 21st-century Thai politics, Thaksin Shinawatra. His next electoral test came in 2007, when he was defeated decisively. The greatest “challenge” of democracy for Mr Abhisit has been as simple as that - whenever they have been given a chance to elect him, Thai voters have chosen someone else.

Thaksin represents another challenge: a profoundly unsavoury politician who is adored by the majority of his own people. As Prime Minister, he used his great wealth to political and personal advantage (last year he and his wife were convicted in absentia of a multimillion-pound property cheat). In southern Thailand he ordered a brutal campaign against Islamic insurgents which left scores of innocent people dead.

Thaksin's version of the war on drugs was to license the police to execute without trial anyone they suspected of being a dealer. But for all of this, he changed for the better the lives of millions of rural Thais.

His cheap healthcare programme gave the poorest people access to affordable medical treatment for the first time ever. A micro-credit scheme allowed many villagers to lift themselves out of subsistence level poverty. But the majority of Thais chose him as their leader, time and again - and after he was forced into exile, and then criminally convicted, they have gone on voting for his political heirs and supporters.

By contrast Mr Abhisit owes his job, not to the will of his people, but to the support of powerful friends - and even they have required a comically large number of attempts to propel their boy to power. First there was the army, which drove Mr Thaksin into exile in a bloodless coup in 2006. Over the course of a year, the generals convened an assembly of tame delegates who rewrote the country's constitution to give Mr Abhisit a better chance of winning. To imagine the election which followed in footballing terms: the Democrat Party was playing downhill, against a team without a striker, in a game refereed by one of their dads. And still Thaksin's side won.

At this point, Mr Abhisit was helped out by a new and sinister force in Thailand - the People's Alliance for Democracy (PAD). At times he has had the decency to appear slightly embarrassed by this mob of yellow-shirted anti-Thaksin activists, led by a rich media owner and apparently supported by the Thai Queen. What exactly the PAD believes in is not easy to pin down, but at heart they want to strip the vote from those silly people who can't be trusted not to vote for Thaksin's side.

When they don't get their way, they resort to force, occupying first the Prime Minister's office and then Bangkok's international airport last year, in chaotic scenes that were broadcast across the world.

The Democrats have never employed such tactics themselves, but they have benefited from them. After the latest pro-Thaksin Government was forced from power by a court ruling last year, they formed a Government by jumping into bed not only with PAD supporters, but even former Thaksin cronies, under the watchful supervision of the army. Mr Abhisit might argue that these were political compromises necessary so that a decent man could finally get his hands on the levers of government. But in the three months since he became Prime Minister, he has come to look more like the puppet than the master of those who hoisted him to power.

A series of disgraceful incidents have made it harder than ever to understand what has happened to the liberalism for which he used to stand. In January, the Thai military beat up and set adrift some 1,000 boat people from Burma, scores of whom died at sea. Journalists and academics continue to be arrested and imprisoned under Thailand's Kafakaesque lèse-majesté law, under which a prison sentence of 12 years can be imposed for dispraise of the Thai King and his family.

At times, it has looked as if someone in power is consciously making a fool of Mr Abhisit - such as the speech he gave last week about the importance of media freedom, which was followed a few hours later by the arrest of the webmaster of an independent website.

Thailand is no Zimbabwe or China, and by comparison with most of their Asian neighbours, Thais are blessedly free and prosperous. But it has the alarming air of a democracy lurching into reverse and out of control, in which familiar freedoms are flying out of the window with unpredictable speed. It is all the more painful that this should be happening under a leader of such obvious talent, a man with all the qualifications except the essential one - democratic legitimacy.

Richard Lloyd Parry is Asia editor of The Times

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