The former leader of Thailand's toppled ruling party on Tuesday launched the first official opposition group to the new military regime, trying to draw together dissidents within the country and overseas.

Jarupong Ruangsuwan, an ex-minister and wanted man in Thailand, said the new "Organisation of Free Thais for Human Rights and Democracy" (FT-HD) would press for the restoration of democratic rule.

Since grabbing power from the elected government in a coup on May 22, the junta has muzzled dissent across the nation and effectively stifled any attempt to coordinate widespread anti-coup actions.

It has imposed strict media controls and summoned and detained hundreds of people, the majority linked with the deposed Puea Thai government of ex-premier Yingluck Shinawatra and her administration's "Red Shirt" supporters.

The anti-coup group, which will operate from an unnamed country, will resist any moves by the establishment-backed junta to rig the political system in its favour, Jarupong said in the group's founding statement.

Decrying the coup as "grand larceny", he accused the junta of violating "the rule of law, abusing democratic principles" and destroying "rights, liberties, and human dignity."

The former interior minister stepped down from his party role last week and is the subject of an arrest warrant after refusing to report to the military.

The organisation is backed by a coalition of former lawmakers, academics, Red Shirt figures and other opponents of the army power grab.

"The people are demanding a fightback," FT-HD spokesman Jakrapob Penkair told AFP from London, where he was speaking on Monday.

"This is just the start. We will provide assistance and encouragement to all demonstrators against the coup... but not of the violent kind."

Jakrapob was once a spokesman for the government of Prime Minister Thaksin Shinawatra, Yingluck's brother, and a founding member of the Red Shirt movement. He said Thaksin had neither been consulted on the group or joined it.

Army chief Prayut Chan-O-Cha has suspended the constitution, assumed sweeping powers and smothered dissent, warning that anti-coup protesters face military courts if prosecuted.

The junta's foreign affairs ministry batted away the FT-HD announcement.

"There is only one legitimate government, that is this administration," permanent secretary Sihasak Phuangketkeow told reporters.


'Thorn in regime's side'

The army has faced a barrage of international condemnation since taking power.

On Monday the European Union halted all official visits to Thailand and suspended the signing of a partnership and cooperation accord.

Analysts doubt whether an overseas-based group can shape Thai politics in the months ahead.

But as a pro-democracy "resistance" group it may spark further international criticism of the army, said Paul Chambers, director of research at the Institute of Southeast Asian Affairs in Chiang Mai.

It could be a "thorn in the regime's side", he added.

The kingdom has suffered a deep political rupture in the years since the emergence of Thaksin, Yingluck's older brother. He swept onto the political centre stage in 2001 on a wave of support from the populous north of the country.

The Shinawatras' electoral success among the rural northern poor and urban working class has shaken the Bangkok-based royalist elite -- and its supporters in the military -- who accuse the family of abusing democracy to sponsor massive graft and cronyism.

Some observers believe the turmoil stems from anxiety among competing elites over who will hold political control after the rule of the nation's revered but ailing 86-year-old king comes to an end.

Thaksin was deposed as premier 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 Yingluck was ousted 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.