The protesters say Thaksin was wrongfully ousted and Abhisit, who was appointed by parliament in December, took power illegitimately.
They are demanding he step down and call fresh elections, calls Abhisit has rejected.
"The rally can continue as long as they act within the framework of the law"
On Thursday, speaking on national television, he urged the protesters not to let themselves be manipulated by Thaksin
"Protesters should not fall victim to people who want to escape conviction. They should not rally in disguise for just one person who will not have to take responsibility," Abhisit said.
The red-shirt protesters have accused the country's elite, the military, judiciary and other unelected officials, of interfering in politics.
Their top target in recent weeks has been Prem Tinsulanonda.
|Abhisit urged the protesters not to be 'manipulated' by Thaksin [AFP]
Thaksin has accused Prem of being behind the coup that toppled him in 2006, and the protesters have issued a 24-hour deadline for Prem to resign.
The accusations against Prem have broken a major taboo in Thai society, where the royal family and their inner circle are traditionally revered.
More than 4,000 police were deployed in the area of the demonstration, with protest leaders vowing to camp out for at least three days.
Abhisit has said that anti-government rallies would be allowed to continue so long as they remained peaceful.
"My standpoint remains the same: the rally can continue as long as they act within the framework of the law," he told a local TV network on Thursday.
Fears of clashes
The protesters are angry about the way Abhisit took power in December, after a court ruling that removed a government made up of Thaksin allies from power.
That ruling came after months of protests by rival, yellow-shirt protesters claiming allegiance to the monarchy, who occupied Government House and organised a blockade of Bangkok's airports late last year.
Leaders of the yellow-shirt protests have said they are considering returning to the streets to challenge the current anti-government protests – raising fears that there could be violent clashes on the streets of Bangkok.
Thaksin, who is living in an undisclosed foreign country to avoid a prison term for corruption, promised that the anti-government protests would mark a "historic day for Thailand".