The Friday evening demonstration was called for by an anti-Thaksin group led by Tul Sithisomwong, the Bangkok Post reported.
Tul called for protesters to gather again Monday and submit a letter to oppose the government-proposed decree that would pardon Thaksin along with 26,000 other convicts for King Bhumibol Adulyadej's birthday, Dec. 5.
"The government should not hold the 26,000 prisoners as guarantors [for the passage of the decree]," said Kaewsan Atibhodi, a former constitution drafter who attended the protest.
Opposition and Democrat Party leader Abhisit Vejjajiva Friday called on Prime Minister Yingluck Shinawatra to avoid a political crisis by pushing the decree forward, saying it would damage the government's credibility.
"It depends on the government and the prime minister. There is no problem if they follow tradition," he said.
Speaking from Dubai Wednesday, Thaksin -- who was sentenced to two years in jail in 2008 for abuse of power after he helped his wife, Potjaman na Pombejra, secure a plot of land -- said he did not know if he was included in the pardon.
"I don't know. I don't think so. No one knows, because it was a confidential meeting. It's at the full discretion of His Majesty," he said.