Bangkok, Thailand ? Anti-government protesters occupying Thailand’s international airport seized a police officer, escalating tensions in a four-day standoff that appeared headed for a violent confrontation today.

The trouble started after about 1,000 demonstrators moved in on a police checkpoint near the Suvarnabhumi international airport, triggering a showdown that ended without violence when officers hastily withdrew.

One policeman remained, however, and Associated Press reporters saw him being grabbed by three protesters, forcibly put in a vehicle, and driven away toward the airport controlled by the demonstrators.

It was not immediately known if the officer was still being held.

Members of the People’s Alliance for Democracy seized the Suvarnabhumi airport on Bangkok’s outskirts on Tuesday night, and the smaller Don Muang airport in the city on Wednesday, in an unexpected twist to their monthslong campaign to oust the government.

The alliance accuses the government of being a puppet of former Prime Minister Thaksin Shinawatra, who was ousted by a 2006 military coup and fled overseas to escape corruption charges. Current Prime Minister Somchai Wongsawat is Thaksin’s brother-in-law.

The capital remains cut off from all civilian air traffic, stranding thousands of travelers and dealing a severe blow to the Thai economy and tourism industry.

After nearly four days, the situation appeared to be heading toward a confrontation as both protesters and police reinforced their presence at Suvarnabhumi airport.

Hundreds more protesters arrived at the airport overnight, boosting their number to several thousand, and ambulances lined up in anticipation of raids by police.

The alliance has also taken over the sprawling compound of the prime minister’s office in the heart of Bangkok.

Alliance leader Chamlong Srimuang attempted to rally supporters gathered in the prime minister’s compound, accusing police of blockading the airport to deny protesters food and water.

“This is urgent. If you want to join us, go help our friends in Suvarnabhumi,” Chamlong said in a speech televised on the anti-government television station ASTV. “We are going to encircle police when they try and shut our friends out from any help.”

Police, many in full riot gear, also had a much more visible presence, guarding an airport hotel and airport management offices, and assembling in a nearby administrative building. There appeared to be several hundred in the area.

But acting National Chief Gen. Pateep Tanprasert insisted they would do everything they could to avert a crackdown on protesters.

“We are following the prime minister’s instruction to end the crisis as soon as possible,” Pateep said. “My strategy is also based on nonviolence. Currently, we are trying to open negotiations with protesters.”

However, a member of the alliance said discussions were not on the agenda.

“Our stance is clear. We do not negotiate,” said Parnthep Wongpuapan, an alliance spokesman. “If police try to break in, we definitely will defend ourselves.”

In downtown Bangkok, about 20 soldiers wearing flak jackets and carrying batons and shields were posted near Victory Monument, a roundabout that hosts a station of the city’s elevated transit system.

A soldier who refused to give his name told The Associated Press the unit was sent out “to monitor the situation in case third parties and ill-intended people decide to get involved.”