With tensions on the rise, pro-government groups have said they will march through the streets of Bangkok in support of Somchai Wongsawat, the besieged prime minister, possibly setting the stage for fresh confrontations.
Meanwhile thousands of travellers have been stranded at the airport, Thailand's main international gateway and an important regional hub, while several governments have warned would-be visitors to avoid any travel to Thailand.
|Key facts: The PAD
Group is a loose coalition of royalists, businessmen and urban middle class -Thailand's traditional elite.
PAD led protests that triggered 2006 coup against the then PM, Thaksin Shinawatra.
Supporters wear yellow shirts, a colour associated with Thailand's revered king.
Group accuses Thaksin supporters of pushing to turn Thailand into a republic, an allegation rejected by Thaksin.
Critics say PAD's contempt for results of three democratic elections show it is neither popular nor democratic.
Click here for more on the PAD
Incoming flights have been diverted to other airports around Thailand and the region, while the director of Suvarnabhumi airport said negotiations were underway with the protesters to allow stranded passengers to fly out.
"The incident has damaged Thailand's reputation and its economy beyond repair," Serirat Prasutanont said.
Protesters from the People's Alliance for Democracy (PAD), some armed with metal rods, stormed the airport on Tuesday night and appeared to be digging in for long siege in an effort to raise the stakes in the confrontation with the government.
The apparent ease with which the protestors took over what should normally be a tightly-guarded airport has raised questions over the degree of support in the Thai military for the protesters cause.
Al Jazeera's Selina Downes, reporting from Bangkok, said the grenade attack early on Wednesday morning was carried out by pro-government supporters - although who exactly is not clear.
Another attack took place on the road leading to the old Don Muang airport, around the area where anti-government protesters shot at opponents in violent clashes a day earlier, our correspondent said.
Somchai Wongsawat, the Thai prime minister, and his cabinet set up temporary offices at the old airport after the grounds of Government House in central Bangkok were occupied by protesters in August.
|Television footage showed PAD security guards firing handguns at opponents [AFP]
The airport protests, involving about 8,000 demonstrators, according to police, are part of a campaign by the PAD, a loose alliance of royalists, academics and businessmen, to unseat Somchai, who is expected to return on Wednesday from a summit of Apec leaders in Peru.
The prime minister is now expected to use another airport or arrive on a military flight to avoid the disruption in the capital.
"Our goal is to shut down Suvaranbhumi airport until Somchai quits," Parnthep Pourpongpan, a spokesman for the PAD, said.
Norman Hermant, a journalist monitoring events at the airport, told Al Jazeera that more and more PAD protesters were arriving by the minute.
He said the protesters had set up a stage and were distributing water, acquiring "a degree of permanence very quickly."
Larry Jagan, a journalist and commentator based in Bangkok, told Al Jazeera that Somchai would likely be forced to take tougher action against the PAD once he is back in the country.
"I think he is going to have to act and the possibility is that he will call a state of emergency, declare martial law, so that the authorities can deal more effectively with the protesters," he said, adding that the key issue was whether the military would step in.
|Thousands of passengers have been stranded at Bangkok's Suvarnabhumi airport [EPA]
The military refused to enter the fray when the former prime minister, Samak Sundaravej, declared an emergency in September and it has made it clear that there will be no coup.
But with pictures of the airport siege being broadcast internationally that could be devastating for tourism and foreign investment, and "the army may feel that they are obliged to move if a state of emergency is called", Jagan said.
"The politics of the future of Thailand is at stake," he said.
"What we're seeing is a battle between the traditional elites who want to return to something like a paternal democracy and the new middle classes under Thaksin [Shinawatra], the former prime minister, who really want to open up Thailand and see a global presence for the country."
Analysts also say the airport siege - the PAD's most disruptive act so far in its four month campaign against the government - could undermine public support for a movement that appears to be going to ever greater extremes to provoke a violent government backlash.
Norm Hermant, a journalist based in Bangkok, told Al Jazeera that the PAD protestors "have not been able to create enough pressure to force the government to resign or force the army to step in. Meanwhile their numbers have been continually declining".
"They have been trying to push for what they call this final showdown. But numbers yesterday showed that it wasn't - they didn't get anywhere near the 100,000 people they were promoting that they would get."
Scores of people have been injured in clashes between demonstrators and government supporters this week.
Television footage showed two security guards from the PAD firing handguns at opponents on a major road in north Bangkok on Tuesday.
But riot police have largely refused to intervene, fearing a repeat of clashes between protesters and police on October 7 that left two people dead and 500 injured, the worst political violence in Thailand for 16 years.
PAD leaders have called the latest protests the "final battle" in their campaign to unseat the Somchai's People Power party (PPP), which has close ties to Thaksin, the exiled former prime minister who was ousted from power in a 2006 coup.
The PAD accuses the government elected in December last year of being tainted by corruption and of being Thaksin's puppet.
Somchai, who is Thaksin's brother-in-law, has rejected calls to step down as prime minister.
The ongoing political crisis has stymied government decision-making and undermined investor confidence in Thailand's economy.
And fears are rising that the airport siege will do massive damage to its $15bn tourism industry still reeling from similar but smaller incidents in August when PAD protesters occupied the airports at popular beach destinations Krabi and Phuket.