The protesters were suspected of possessing hidden weapons for protection against a potential attack from pro-government activists, the Thai News Agency reported Saturday.
Retired Maj-Gen. Chamlong Srimuang of the People's Alliance for Democracy said authorities would have found weapons by now as they have been monitoring protesters for two weeks.
He added that protesters only carry flag poles, baseball bats and shields because "the police couldn't protect us."
Alliance activists were participating in ongoing protests Saturday outside the United Nations Economic and Social Commission for Asia and the Pacific main office in Bangkok, the report said.
It is reported pro-government activists continued to hold demonstrations nearby.