Television footage showed two security guards from PAD firing handguns at opponents on a major road in north Bangkok.
The PAD has staged a six-month long protest to get the Thai government to resign.
The clashes broke out after government supporters threw rocks at a truck carrying members of the anti-government PAD along a busy road, the AFP news agency reported.
"The protests have been contained to just two parts of the Thai capital," Selina Downes, Al Jazeera's correspondent in Bangkok, said, refering to the Suvaranbhumi airport protest and demonstrations at Bangkok's old on Mueang international airport where Somchai is temporarily based.
|Television footage showed PAD security guards firing handguns at opponents [AFP]
"They certainly haven't spread across the country ... But some people are now saying the military needs to step in because the government is losing control."
Riot police have largely refused to tackle protesters amid fears of 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 six-month campaign to unseat the People Power party (PPP), which has close ties to Thaksin Shinawatra, the exiled former prime minister who was ousted from power in a 2006 coup.
The PAD – a loose alliance of royalists, academics and businessmen - 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.
Thousands of PAD supporters earlier rolled out razor wire across a four-lane access road to Bangkok's Suvarnabhumi airport and waved plastic hand-clappers, flags and portraits of King Bhumibol Adulyadej, Thailand's monarch.
The airport siege has been one of the PAD's most disruptive acts in its six-month campaign against the government.
Analysts say the move could undermine public support for a movement that appears to be going to ever greater extremes to provoke a violent government backlash.
"It is time to make a clear-cut choice between good and evil, between those who are loyal and traitors," Somsak Kosaisuk, a PAD leader, told supporters at a rally earlier in the day.
The protest came a day after PAD supporters surrounded the Thai parliament building, forcing MPs to postpone a joint session.
|The airport siege has been one of the PAD's most disruptive acts [AFP]
Tensions soared last week after a demonstrator was killed and several others were injured in a grenade attack on a PAD protest camp in the grounds of Government House.
Unions had said they would call a nationwide strike on Tuesday if the government did not quit, but the threatened walkout did not materialise.
Norm Hermant, a journalist in Bangkok, told Al Jazeera: "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.
"So they have been trying to push for what they call this final showdown.
"But numbers yesterday showed that it wasn't a final showdown. They didn't get anyway near the one hundred thousand people they were promoting that they would get."
The ongoing political crisis has stymied government decision-making and undermined confidence in Thailand's export-driven economy, which has also been hit by the global financial crisis.
According to the latest government data the Thai economy will grow at 4.5 per cent this year, its slowest rate in seven years – due both to slumping investment and a slide in exports.