“Hundreds of them are inside parliament premises,” said Pitoon Pumhiran, secretary general of the House of Representatives.
“They used a truck to break the gate, but so far there is no violence yet.”
About 150 police officers were guarding parliament, he said.
When he learnt that the reds were approaching, Prime Minister Abhisit Vejjajiva left a cabinet meeting there for a military barracks in the city’s northern outskirts, where he has been based for much of the weeks-long protest.
The cabinet extended a tough security law Wednesday as protesters refused to leave Bangkok’s main commercial district, where they have been since Saturday, and vowed to march to the premier’s house.
Call for fresh elections
About 5,000 Red Shirt demonstrators, supporters of fugitive former premier Thaksin Shinawatra, massed outside the parliament on Wednesday as part of their rolling rallies aimed at forcing immediate elections.
Parliament president Chai Chidchob declared the lower house meeting closed shortly after 11am (1400 AEST) because of the low number of politicians present.
The demonstrators are mostly from Thailand’s poor and working class and see Abhisit’s government as elitist and undemocratic.
Government spokesman Supachai Jaisamut said the authorities were ready to use emergency law if needed.
“If the situation deteriorates, it’s necessary for the government to invoke emergency rule,” he said.
PM’s US visit cancelled
Abhisit cancelled a planned trip to the United States for a nuclear security summit next week due to the unrest.
The Reds have been emboldened after the police and army backed down on Tuesday following a tense standoff in the capital’s tourist heartland.
Thai authorities have threatened the protesters with a year in jail but so far no arrests have been made.
Security forces have refrained from using force to disperse the tens of thousands of protesters, who have disrupted traffic and caused major shopping centres to shut.
The government said it would act if needed to end the protests, but reiterated that it wanted a peaceful resolution to the standoff.
The special security law, extended for another two weeks, allows authorities to set up checkpoints, impose curfews and limit movement.
Protesters clash with police
The Reds have threatened to parade by truck, car and motorcycle to parliament and then to Abhisit’s house, which was targeted by protesters last month throwing bags of their own blood.
The Reds say the government is illegitimate because it came to power with army backing through a parliamentary vote in December 2008 after a court decision ousted Thaksin’s allies from power.
On Tuesday protesters threw plastic bottles, pushed against police barricades and later took over the streets of central Bangkok on motorcycles and in pick-up trucks, pouring into the capital’s financial district.
The military has mounted a heavy security response, deploying 50,000 personnel at one point to try to contain the protests, which drew as many as 100,000 people on the first day on March 14.
But the government wants to avoid a repeat of last April’s clashes with Red Shirts that left two people dead, six months after riot police took on the rival Yellow Shirts in bloody scenes outside parliament.