The last major protests, in April last year, turned violent, with two deaths and dozens of people injured.
The red shirt movement, led by the United Front for Democracy against Dictatorship (UDD), say they will rally until Prime Minister Abhisit Vejjajiva calls new elections.
The protesters say Prime Minister Abhisit came to power illegitimately with the backing of the military and the Bangkok-based elites.
Mr Thaksin's main power base was in the rural north. The BBC's Rachel Harvey - in north-eastern Udon Thani province - says people there have neither forgotten nor forgiven his removal.
Mr Thaksin, who was ousted amid allegations of corruption and abuse, is now living in self-imposed exile.
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