The United Front for Democracy against Dictatorship will next week send a letter to the United Nations, all foreign embassies and all ministries inviting them to send representatives to observe its centres monitoring fraud in the Aug 7 charter referendum.
UDD chairman Jatuporn Prompan said on Sunday the centres were in Bangkok and other provinces.
Speaking on his television talk show on Peace TV on Sunday, Mr Jatuporn called for the National Council for Peace and Order (NCPO) not to use force to intimidate people at the UDD centres in the provinces when they open on June 19.
The UDD opened its main Bangkok centre at Imperial Lat Phrao department store last week.
Mr Jatuporn claimed Maj Gen Sansern Kaewkamnerd, the government spokesman, and Col Piyapong Klinpan, an NCPO spokesman, had cited an NCPO order prohibiting assemblies of five people or more as a means of warning the UDD not to set up similar centres in the provinces.
He dared the NCPO to close down its main centre in Bangkok, instead of intimidating UDD people in the provinces.
The UDD will send a letter to the Bangkok office of the United Nations, all foreign embassies and all ministries asking them to observe the operations of the UDD centres.
"The prime minister has said that the setting up of fraud-monitoring centres is not against the law. More importantly, the centres are not for a political assembly but to monitor possible fraud in the charter referendum. They will help persuade the people to vote in the referendum according to their personal judgments," Mr Jatuporn said.
Nattawut Saikuar, a UDD core member, said the NCPO staff should listen to Prime Minister Prayut Chan-o-cha who had said clearly that the UDD could open such centres as long as they did not violate the law.