The group joined the rally at about 10pm and Arnon took to the stage to demand that the Constitution be amended to cut down on the power of senators and called on political parties to support the people instead of a dictatorship.
He then marched with the group of 31 activists to Samranrat Police Station to seek clarification about their charges.
However, Arnon said the police were not able to provide a clear explanation or arrest warrants in relation to the July 18 protest.
The group dispersed at around 11.30pm while Pol Maj-General Somprasong Yentuam told media that police were thinking of slapping them with new charges.