UPDATE 4-Thai protesters hit police, tighten grip on airport

(Recasts with latest checkpoint incident, tourism warning)

By Ed Cropley

BANGKOK, Nov 29 (Reuters) - Anti-government protesters tightened their grip on Bangkok's international airport on Saturday, attacking police checkpoints aimed at stopping more people from joining the blockade.

The protests, aimed at forcing out Prime Minister Somchai Wongsawat, have paralysed flights, stranded thousands of passengers and sparked rumours of a military coup, even though the army chief has said he will not seize control.

The government said tourism was suffering and the number of visitors to Thailand could fall by half next year.

In the latest clash, about 150 riot police fled their checkpoint near Suvarnabhumi airport after they were assaulted by protesters hurling iron rods and firecrackers from speeding cars.

The People's Alliance for Democracy (PAD) movement, which invaded the airport four days ago, then stationed guards on the expressway exit to prevent the police returning.

Earlier, about 2,000 PAD members forced riot police to abandon another checkpoint near the airport. There was no violence, but one police officer was detained by PAD "security guards", the Nation newspaper reported on its website.

The PAD's occupation of Suvarnabhumi, and a second older airport in Bangkok, is a dramatic escalation of their six-month street campaign against Somchai.

The airport closures have crippled the tourism industry during the peak end-of-year season. Somchai, who has refused to quit, imposed emergency rule at the airports two days ago but police have made no moves to evict the thousands of protesters.

Somchai, who is running the government from its political stronghold in the northern city of Chiang Mai, demoted his national police chief on Friday. While no official reason was given, Thai newspapers said he had been sacked for refusing to send riot police in to end the protest.

"SHOOT THEM BACK"

The PAD, a coalition of royalist businessmen, activists and academics who accuse Somchai of being a puppet of his brother-in-law, ousted prime minister Thaksin Shinawatra, seized the airports in a "final battle" to unseat the government.

At Suvarnabhumi, PAD youths armed with iron stakes and wearing police riot helmets manned barricades, scanning with binoculars for signs of police or pro-government gangs.

"If they come, we'll not open the door. If they shoot us, we'll shoot them back. We'll die if that makes the country better," PAD leader Sondhi Limthongul told supporters, the most explicit admission yet by the movement that they are armed.