This time, it was Thaksin loyalists instead of his opponents who took to the streets.
"Yes, we will move to Parliament. But we will allow MPs to go in and out tomorrow," a protest leader, Korkaew Pikunthong, told The Associated Press.
The alliance - dubbed the "red shirts" for their favored protest attire - says the new Prime Minister Abhisit and his Democrat Party came to power this month through a virtual coup d'etat.
The group - which calls itself the Democratic Alliance against Dictatorship - says the court ruling that dissolved the previous government, which was packed with Thaksin allies, and led to Abhisit's selection as prime minister came under pressure from the military and other powerful forces.
Police closed the gates of the Parliament building Sunday in anticipation of the demonstrations. The new government plans to deliver its policy statement to the legislature Monday and Tuesday.
Police lines were reinforced in an effort to cordon off the Parliament building and Sanam Luang, a field in the historic heart of the capital where the pro-Thaksin group gathered Sunday to hear speeches denouncing the government.
An estimated 10,000 rallied at the field while several hundred gathered around Parliament Sunday night. Protest leaders at Sanam Luang said they would move to Parliament and camp there overnight.
Abhisit told reporters that force would not be used against the demonstrators.
Earlier, police Maj. Gen. Amnuay Nimmano said police would avoid any clash with the protesters but that if the rally veered toward violence, its organizers should disperse the crowds.
Warong Dechgitvigrom, a spokesman for the ruling Democrat Party, said party representatives would go together to Parliament on Monday morning and if it was blocked they would return to party headquarters. He said the government did not plan to force its way into the building.
An Oxford-educated, 44-year-old politician, Abhisit was formally named prime minister Dec. 17 in what many hoped would be the end of months of turbulent, sometimes violent, protests that had their roots in a 2006 military coup that toppled Thaksin.
Thaksin and his backers retain strong support in rural areas but have lost ground recently as former loyalists defected to join Abhisit's government, behind which the powerful military and monarchist figures have thrown their weight.
Forced out of England where he sought exile, facing probable imprisonment should he return to Thailand, Thaksin no longer seems the prime mover in Thailand's political arena -- although some still don't count him out.
Local media has speculated that Thaksin, once Thailand's richest man, has also taken heavy losses in the current financial crisis and no longer has the seemingly bottomless purse to support, and motivate, his backers.
Abhisit, the nation's third prime minister in four months, vowed in his inaugural address to reunite the deeply divided nation and to restore Thailand's tourist-friendly image. The eight-day airport shutdown battered the country's essential tourism industry and stranded more than 300,000 travelers.
Abhisit's Democrat Party had been in opposition since 2001, when Thaksin, a former telecommunications tycoon, first came to power in a landslide election.
Military leaders ousted Thaksin in September 2006, accusing him of corruption, keeping him in exile and controlling the country for an interim period until new elections in December 2007 brought Thaksin's allies back into power.
He returned to Thailand in February 2008 to face corruption charges but later fled into exile again and was convicted in absentia.
Thailand's recent political convulsions began in August when anti-Thaksin protesters took over the seat of government to demand that Thaksin's allies resign. Since then, a series of court rulings resulted in the ouster of two Thaksin-allied prime ministers.
In October, street clashes with police outside Parliament left two people dead and hundreds injured.