But instead, the dramatic 4-minute assault effectively broke the cordon around the airport, which protesters overran Tuesday night as part of their campaign to force Prime Minister Somchai Wongsawat from office.
Bangkok's domestic airport has also been seized, severing the capital from all commercial air traffic and virtually paralyzing the government.
The closure of the airports has taken a heavy toll on Thailand's economy and reputation. According to Thai media reports, some 100,000 tourists are stranded, and schedules of airlines around the world have been disrupted.
The protesters, carrying metal rods and some guns, smashed windshields and threw what appeared to be firecrackers at the police. Video footage of the attack appeared to show a protester firing a handgun toward a police van filled with officers.
Police Col. Wuttipong Petchkumnerd said there were no injuries on either side.
"We left the area immediately because we did not want any confrontation," he told The Associated Press.
"The police are constantly provoked, which is why only senior policemen are armed. We do not want to use violence," he said. He said four police trucks were damaged.
So far security forces have only issued a warning to the protesters to leave. It was not clear if the assault will result in a changed strategy.
Earlier Saturday, Police spokesman Maj. Gen. Amnuay Nimmano said the protesters would be told to leave the airports. If they did not, a deadline will be issued with another warning, "the last one before we take action," he said.
The protesters, who call themselves the People's Alliance for Democracy, accuse 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.
In another sign the standoff could become more violent, an explosion was reported early Sunday morning inside the Somchai's office compound, which protesters seized in August and have held ever since. At least 33 people were wounded, including five who were hospitalized, according to a spokesman from the Narentorn Medical Center. He spoke on condition of anonymity in line with policy.
An Associated Press photographer said something was hurled from outside the compound, but it was not clear what was thrown. The protest group has been attacked several times by small bombs and grenades, including a blast earlier this month that killed one person and wounded 29.
The Federation of Thai Industries has estimated the takeover of the airports is costing the country $57 million to $85 million a day.
The airport authority said Saturday that Suvarnabhumi international airport will remain closed at least until Monday evening.
Several airlines have begun flying rescue flights to the U-Tapao naval airport, 140 kilometers (90 miles) south of Bangkok, to evacuate stranded passengers. But the small airport is overwhelmed by the load, unable to process thousands of travelers quickly.
Among those stranded are about 3,000 Chinese tourists who will be flown out on special flights by four Chinese airlines beginning Saturday, China's official Xinhua News Agency reported. It said the Hong Kong government has also arranged two Cathay Pacific flights to help stranded passengers.
Thailand's central bank said the number of tourist arrivals is likely to fall by 40 percent next year if the airport shutdown drags on until the end of December. It said the tourism industry, a key component of the Thai economy, is expected to lose $4.28 billion, equal to 1.5 percent of the gross domestic product.
With international repercussions obvious, the European Union and the United States urged the protesters to end their siege.
Tension rose further Saturday after a pro-government group expressed frustration at the continuing standoff and called for an indefinite sit-in starting Sunday in central Bangkok.
Associated Press reporter Ambika Ahuja contributed to this report.