BANGKOK (AFP) - Thailand's army rulers will appoint a national assembly stacked with military officers to pick an interim government leader, officials said on Thursday, as they seek to retain their influence over the kingdom's political transition.

In the first real hint of the shape the politically fraught country's next administration may take, army sources told AFP that the military will select the 200 assembly members and that the junta itself will not be dissolved.

"We have learned our lesson. By pushing power in other people's hands, they may not do what we expect them to do," said an official under the condition of anonymity.

The kingdom's generals are keen to avoid ceding as much power to the interim government as they did following the last coup in 2006.

Earlier this month junta chief General Prayuth Chan-O-Cha said the regime would set up an interim government by September to oversee political reforms, including crafting a new constitution, followed by elections in about a year's time.

Pro-coup demonstrators have called for reforms that would rid the country of the influence of the Shinawatra family, whose political parties continually win during elections but are loathed by much of the country's powerful elite.

The junta - formally known as the National Council for Peace and Order (NCPO) - has now finished drafting an interim constitution, according to the military official.

The comments were confirmed by another army source who also requested anonymity.

"The interim constitution is complete... The NCPO's powers will remain - different from the 2006 coup when the military lost power after establishing an interim government," the source said.

The officials did not specify the exact balance of power between the junta and the planned interim government.

The remarks came as a Thai opposition alliance set up to counter the junta said on 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.

The new anti-coup "Organisation of Free Thais for Human Rights and Democracy" is in talks with several countries in the West over setting up headquarters, spokesman Jakrapob Penkair told reporters in Hong Kong.

General Prayuth, who is due to retire as army chief in September, has not ruled out becoming prime minister himself.

He also has not revealed whether the cabinet, handpicked by the premier, would be made up of civilians or military personnel.

A committee to draft political reforms will also be appointed by the junta and comprise of 250 members, the sources said.

The junta claims that Thailand - which has seen 19 failed or successful coups in recent decades - needs a strong military to help steady the country after months of violent protests between opposing political camps.

In a BBC interview published on Thursday, a senior Thai general rejected reports the army had been planning a coup for several years in a bid to rid the kingdom of the influence of controversial former premier Thaksin Shinawatra.

"So far as I know there was no advanced planning, because if it were planned that would be illegitimate," Lieutenant General Chatchalerm Chalermsukh told the British broadcaster.

"If you're wondering why this (the coup) happened so smoothly, that was because forces were already deployed in the city (Bangkok)," he claimed.

His comments came after local media reported that Suthep Thaugsuban, the leader of mass protests which crippled the former administration, said he had been advising General Prayuth on how to tackle Thaksin since 2010.

The Thai military seized power on May 22 after nearly seven months of protests saw 28 people killed and hundreds of others wounded, paralysing the government of former prime minister Yingluck Shinawatra - Thaksin's younger sister.

It was the latest chapter of a seemingly-intractable political crisis that broadly pits billionaire Thaksin - a fugitive former premier who was toppled by the 2006 coup - and his supporters against a royalist establishment backed by parts of the military and judiciary.

Thailand's rulers have suspended democracy since seizing power, imposing martial law, banning public rallies and censoring the media to stifle any dissent.

The junta has also summoned and detained several hundred people, the majority linked with the deposed Puea Thai government and the Shinawatra family's "Red Shirt" supporters.

On Wednesday, the junta said it would set up five panels to monitor national and international news organisations as well as social media in its latest attempt to control the press.