Three politicians from the party ousted by Thailand's generals were charged on Monday with sedition for holding a press conference, as dissent grows before the fourth anniversary of the junta's 2014 coup.
The military filed a case after senior Pheu Thai figures railed against the coup organisers at their party's headquarters 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 for speaking at the press conference, while five other party leaders were charged with violating the ban on gatherings for attending.
"They were summoned to hear charges that were filed by the (junta) on Thursday," said deputy national police chief Srivara Rangsibhramanakul.
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.
- Coup anniversary -
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, which is led by a dedicated cohort of student activists, said they have come under increasing surveillance before their planned march.
In a statement the activists said their sound technician was taken to a military camp on Sunday night.
He was released on Monday, according to a group of human rights lawyers, who say 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.