The military filed a case after senior Pheu Thai party figures railed against the coup organisers at a press conference last week, blasting the generals for breaking repeated promises to restore democracy.
The party supports former premiers Thaksin Shinawatra and his sister Yingluck, whose elected governments were both toppled by coups but who remain popular among many rural Thais despite both being in self-exile.
The military now says it will allow elections in February but has refused to lift its ban on politics or on gatherings of more than five people.
But critics are increasingly testing the limits of the ban, which they say is intended to give the generals time to build a political coalition before the vote without any public debate.
On Monday Pheu Thai’s Watana Muangsook, Chaturon Chaisang and Chusak Sirinil were all charged with sedition, while five other party leaders were charged with violating the ban on gatherings, the deputy national police chief told reporters.
“They were summoned to hear charges that were filed by the (junta) on Thursday,” said Srivara Rangsibhramanakul, adding police would not detain the men while the case proceeded.
The Pheu Thai veterans kept up their criticism of the military as they cut through a crowd gathered outside the police office and shouting “Fight, fight!” in support.
“This government abuses the laws. They use laws to prevent people from investigating (them),” said Pheu Thai’s secretary general Phumtham Wechayachai.
The regime is taking a harder line as it gears up for protests on the coup’s fourth anniversary on Tuesday, with activists vowing to march on Government House in the morning.
Police have warned they will enforce a “restricted area” around the building.
The pro-democracy movement led by a dedicated cohort of student activists said they have come under increasing surveillance before their planned march.
In a statement the group said their sound technician was summoned to a military camp on Sunday night and had not been seen or heard from since.
According to a group of human rights lawyers, more than 50 people have been visited, called or monitored by security officers since May 17.
A junta spokesman could not be reached for comment.
Titipol Phakdeewanich, a politics professor based in Thailand’s rural northeast, said officers had been visiting local activists in the region, a Pheu Thai stronghold.
“They are also using their network in local areas as well… having some of their military going into villages and asking people not to go (to the Bangkok demonstration),” Titipol told AFP, adding that he himself received a phone call from the military asking if he planned to attend the march.