The protest comes a day after Mr Abhisit's motorcade was attacked following a cabinet meeting in the resort town of Pattaya.
The demonstrators, from the United Front for Democracy against Dictatorship (UDD), accuse the current government of being a puppet of the military.
Protest leaders are calling Wednesday their "D-Day" in their efforts to push Mr Abhisit to dissolve his four-month-old government and hold elections.
Mr Thaksin, who is living in an undisclosed foreign country, said late on Tuesday that the protests would mark an "historic day for Thailand".
"We will come peacefully but we need as many people as possible to show that the Thai people will not tolerate these politics any more," he said in a speech via video link to supporters outside Government House.
Plea for calm
British-born Mr Abhisit came to power in December after a court ruling removed Thaksin's allies from government.
The pro-Thaksin lobby is on Bangkok's streets, calling for fresh elections
At the time, yellow-shirted anti-Thaksin protesters had closed the country's airports for a week.
The Thai government is preparing to host leaders of 16 Asian nations from 10-12 April in the coastal resort of Pattaya.
Mr Abhisit said the meeting would go ahead as planned, despite the demonstrations.
"There's a group of people wanting to create chaos, but the government will do everything to restrain them," he told local television.
"If there's rioting, we will have to do something. I can affirm there will be no violence starting from the government's side."
An earlier summit of the Association of South East Asian Nations had to be postponed due to the political protests which helped usher Mr Abhisit into power.
The country remains deeply divided between Mr Thaksin's followers among the urban and rural poor and his foes in the traditional power cliques of the military and bureaucracy.
The Bank of Thailand has cut its key interest rate by 25 basis points to 1.25%, the fourth reduction in four months as the country struggles with a weak economy.
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