The PAD supporters want to evict Somchai Wongsawat, the Thai prime minister, whom they say is merely a front for his brother-in-law, Thaksin Shinawatra, the former leader who was ousted in a 2006 coup and now lives in exile.
Officials said that once the protest ends, it could take a week to get the main airport up and running again because security and computer systems had been compromised during the blockade.
"Normally, checking the IT systems takes one week," Serirat Prasutanond told Reuters, but it will take longer if there were repairs to the airports massive computer systems.
"We have to check, recheck, check, recheck," he added.
On Monday thousands of government supporters wearing red shirts, headbands and bandanas held a rally in the capital, in the first show of strength by the Democratic Alliance Against Dictatorship (DAAD) since the PAD's "final battle" against Somchai.
The DAAD's move, which raised the stakes in the country's political future, came ahead of a ruling in a vote fraud case that could deliver a crippling blow to the six-party coalition government.
"This is a movement against anarchical force and the people behind it," Nattawut Sai-Kua, a government spokesman, told The Associated Press.
"They want anarchy so that the army is forced to intervene and stage a coup."
Thailand's constitutional court on Tuesday is expected to wrap up the case in a decision many expect will be to order the disbanding of the ruling People's Power Party and its partners.
Late on Saturday a grenade attack near a stage set up for rallies in front of Government House wounded at least 50 protesters who have occupied the prime minister's office since August.
A series of grenade attacks on the PAD camp at Government House last week killed two protesters. No one has claimed responsibility for any of the incidents.
|Protesters continue to defy police orders
to end the airport siege [EPA]
Meanwhile the Federation of Thai Industries has estimated the airports takeover is costing the country $57m to $85m a day.
Andrew Stotz, a security analyst, told Al Jazeera that the Thai economy will be hit hard.
"The current situation will hit tourism, business investments, and those trying to export and import goods on a consistent basis," he said.
"Thailand has had an economy that's been on the decline for a long time. While the country has had many political crises, we are at the peak of this current quagmire."
The airport siege, coming in the wake of a global economic slump, has also hurt Thailand's $15bn tourism industry.
The government estimates that the number of visitors could be cut by half if the crisis drags on, leaving about one million Thais jobless.
But PAD protesters see that as a price worth paying.
"Obviously it hurts the economy, but it's the only way we can push out this government. We have to sacrifice something," Prathan Tandavanitj, 60, told Reuters at the airport.