Speakers at the rally on Sunday also accused Samak of trying to change the country's constitution as a way to hold on to power and to prevent Thaksin from facing justice.
Thaksin faces corruption-related charges filed by special committees set
up by the military after the coup.
Thailand will hold a referendum in early July on whether to amend the military-backed constitution, which was approved by voters less than a year ago.
If the constitution is amended to deprive those committees of their legal
basis for existence, charges brought by them against Thaksin and his family could in turn be abandoned.