A Thai opposition alliance set up to counter the military regime installed in a May coup said Thursday it would establish an official base in a Western country by next month.

Thailand's junta has muzzled dissent, summoning and detaining hundreds of people, the majority linked with the deposed government of ex-premier Yingluck Shinawatra and her administration's "Red Shirt" supporters.

Jarupong Ruangsuwan, who was leader of the Puea Thai party as well as a senior minister, will lead the new anti-coup "Organisation of Free Thais for Human Rights and Democracy" from self-exile, the group announced earlier this week.

Now it is in talks with several countries in the West over setting up headquarters, with the location to be decided in July, spokesman Jakrapob Penkair said in Hong Kong Thursday.

"We have not made a final decision on the place where the headquarters will be, but we have been in talks with more than five countries, all in the Western hemisphere," said Jakrapob, a former government minister and founding member of the Red Shirt movement, speaking at the Hong Kong Foreign Correspondents' Club.

There had been speculation that the group might base themselves in Hong Kong or Cambodia, but Jakrapob ruled out an Asia headquarters.

"It's not to say the West is superior, but it is more established, it's more well aware of the tricks and the games being played in Thailand now.

"I believe that by July we should be able to say where this office will be," he added, saying it would be clearly visible and open to the public.

"We have to work from outside... but eventually changes must be made from inside, that's what we're working towards," he said.

A coalition of former lawmakers, academics, Red Shirt figures and other opponents of the May 22 army power grab, have also joined the group.

Jakrapob said former premier and Red Shirt hero Thaksin Shinawatra had not joined the group.

The Shinawatras' electoral success sent panic through the Bangkok-based royalist elite – and its supporters in the military – who accuse the family of abusing democracy to sponsor massive graft and cronyism.

Thaksin was deposed in a 2006 coup and lives in self-exile to avoid jail for a corruption conviction, although his political affiliates have continued to sweep the polls.

His sister was deposed by a controversial court order shortly before May's coup, which the army said was necessary to restore order following several months of sometimes-deadly street protests in Bangkok.