The Foreign Office currently warns all Britons travelling to Saudi Arabia to exercise vigilance: "Any increase in regional tension might affect travel advice," its website says. It is a warning that David Cameron could do well to heed when he flies to the kingdom today.

Saudi Arabia might seem an ideal customer to a British prime minister keen to win contracts. If Barack Obama can sell the kingdom nearly $30bn of F-15 fighter jets, Britain can surely flog its armoured personnel carriers, sniper rifles, small arms ammunition and weapon sights.

On one side of the kingdom, Iran is planning to hold another round of military exercises in the Strait of Hormuz. The naval commander of the Revolutionary Guards, Real Admiral Ali Fadavi, was quoted by Fars news agency as saying that the Islamic Republic has full domination over the region and "controls all movements within it".

On the other side, Bashar al-Assad and his family are locked in a grim fight to the death with an armed opposition in Syria. The kingdom can easily today promote itself to the credulous or venal as a stable ally in a turbulent region.

However, Saudi Arabia and the Gulf states are fast learning to play another role in the region. The kingdom is branding itself as a bulwark not just against the Revolutionary Guards in Iran and despots in Syria, but against the Arab spring itself.

The kingdom is nervous for two reasons. Externally, the wave of unrest that has swept up to its borders is changing the footprint of regional powers. Saudi Arabia is jealous of Turkey's role as a mentor of political Islam to the emerging democracies of Tunisia and Egypt. And the Turkish model with its blend of Islam and secularism is markedly different from the traditional Saudi one. Grassroots Islamists threaten monarchies, and Turkey is now described in the kingdom as a bigger regional nuisance than Iran. Bilateral relations between Turkey and Saudi Arabia have become strained and negotiations on a free trade agreement between Ankara and the Gulf states were recently frozen.

Internally, the most authoritarian regime in the Arab world has much to fear from demonstrations – which are illegal. Sporadic protests are not confined to the oil-rich eastern province where the minority Shia community live. The king's response has been to blend small reformist steps, such as the decision in September to allow women to vote and run in municipal elections (but not yet to drive), with larger but less visible acts of repression.

In November, 16 men including nine prominent reformists were given sentences ranging from five to 30 years in the specialised criminal court. Their crime? They wanted to set up an NGO. They were convicted of forming a secret organisation, attempting to seize power, incitement against the king, financing terrorism and money laundering.

Another measure is a proposed counter-terrorism law. Last year, Amnesty International published a leaked copy of a draft law that would allow peaceful dissent to be prosecuted as a terrorist crime.

Rothna Begum, Amnesty's campaigner on Saudi Arabia said: "The definition would allow it to criminalise dissent. The embassy wrote to us in September to say that several articles of draft had been amended and some deleted, but we don't know which these were and what stage this legislation has reached. The version we have seen allows the detention without trial of suspects to be extended indefinitely."

Begum said that last year Amnesty had documented a new wave of repression in the kingdom with hundreds being arrested. She said: "It would be good if Cameron could raise specific human rights concerns and not just use the issue as a tick box. For instance, demonstrations are completely banned in the kingdom and it would be good if he was to push for this to be changed."

How does today's visit sit with what Cameron said about the Arab spring in the UN last September: "As people in north Africa and the Middle East stand up and give voice to their hopes for more open and democratic societies, we have an opportunity – and I would say a responsibility – to help them,"?

The answer is: oddly.

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