The protest moved from Chamai Maruchet Bridge to Suan Misakawan Junction, where police had closed traffic.
The rally came as the bill, which includes the legalisation of casinos, was set for Cabinet consideration today.
Meanwhile, Finance Minister Pichai Chunhavajira said before the meeting that the ministry would propose the bill and a draft emergency decree amending the Securities and Exchange Act.
Details will be announced after the meeting, he said.
The entertainment complex bill, which allows entrepreneurs to set up a man-made destination with a casino plus at least four other businesses, is expected to stimulate the economy and tourism, as well as draw foreign investments to Thailand.
However, several academics have expressed concerns over the policy so far, claiming that the legalisation of casinos could lead to gambling addictions, grey businesses and corruption.