BANGKOK - Anti-government protesters besieged Bangkok's main airports on Thursday, forcing flight cancellations and stranding travellers in the latest twist in a six-month street campaign to unseat the government.

The People's Alliance for Democracy (PAD) movement has vowed to stay until Prime Minister Somchai Wongsawat quits, which he has refused to do.

The following Q and A examines some of the main issues:

Police are desperate to avoid a repeat of Oct. 7, when two people were killed and hundreds injured in clashes outside parliament, and so simply melted away when the protesters approached the $4 billion Suvarnabhumi airport on Tuesday.

The PAD are armed and were happy to shoot at police lines last month, suggesting any attempt to remove them by force could result in scores of casualties, increasing the chances of military intervention.

Other possible reasons for police inaction range from incompetence to orders being quashed by bigger forces such as the army or palace.

How does the airport siege help PAD's cause?

The chaos is costing the PAD public support, especially as tourism, which employs 1.8 million people, will suffer badly.

But its ultimate goal is to make Bangkok ungovernable and trigger a putsch against a government they say is a pawn of ousted and exiled leader Thaksin Shinawatra.

Under an interim military government, the PAD would then have more chance of advancing its "new politics" agenda to ensure a parliament stuffed with appointed grandees.

Some of the PAD's plans are code named "Hiroshima" and "Nagasaki", and their ideologues have been quoted on the need for political assassinations.

Who is banking PAD?

The alliance of royalist businessmen, academics and activists says it gets 1 million baht ($42,800) a day from the public.

Analysts suspect it is also bankrolled by anti-Thaksin business interests, parts of the army and palace figures, including Queen Sirikit, who attended the funeral of a PAD supporter killed in clashes with police.

What's Thailand's king got to do with the crisis?

Officially, nothing, but you can't ignore a man seen as semi-divine by nearly 65 million people. As shown during a 1992 uprising, King Bhumibol wields enough moral clout to remove a prime minister, and, by his own admission, when it comes to politics he is "in the middle and working in every field".

There are increasing concerns about his health after three weeks in hospital a year ago with a blood clot on the brain.

Is there a chance of another military coup?

Never rule it out in a country that has had 18 coups or attempted coups in 76 years of on-off democracy, even if top brass say they wouldn't dream of it.

Would a snap election solve anything?

Short-term yes, long-term no. If street violence intensifies - one PAD supporter was dragged from his car and shot dead on Wednesday - Somchai could defuse things by calling a snap poll.

However, lingering support for Thaksin in the countryside would be almost certain to return a broadly pro-Thaksin government, taking everybody back to square one.