Thai army chief Prayut Chan-O-Cha reads a statement after receiving a royal command during a ceremony to swear him in as prime minister at the army headquarters in Bangkok on Monday.

AFP/Bangkok

Thailand's coup leader was formally endorsed as prime minister by the nation's revered king on Monday, in a step towards forming a government to oversee sweeping reforms in the politically turbulent kingdom.

Army chief General Prayut Chan-O-Cha, 60, who booted out an elected government in a bloodless power grab on May 22, was chosen uncontested as premier last week by a national assembly made up mainly of military figures.

In a brief but elaborate ceremony to receive the royal command, Prayut - wearing a white official uniform - knelt and bowed in front of large portrait of King Bhumibol Adulyadej.

"His majesty the king has appointed me Prime Minister. I am extremely grateful... this is a great honour for me and my family," Prayut said after the ceremony.

"I will work with honesty and for the benefit of people and the nation," he added.

The top general, who was endorsed as premier on Sunday but was officially given the royal command a day later, will also remain head of the junta as the military appears to tighten its grip on power.

A junta spokesman said Prayut will pick a cabinet and propose ministers for the king's approval in September.

The junta, formally known as the National Council for Peace and Order (NCPO), says it was forced to take power after months of protests left nearly 30 people dead and hundreds more wounded, paralysing the government, cramping the economy and frightening off tourists.

It has ruled out holding new elections before October 2015, despite international appeals for a return to democracy, vowing first to oversee reforms aimed at cleaning up politics and society.

Critics say the protests provided a pretext for the power grab from the former government, which was led by Yingluck Shinawatra until shortly before the May 22 coup.

Prayut, whose term as army chief finishes at the end of September, is seen as a staunch opponent of Yingluck's brother, fugitive former premier Thaksin Shinawatra, whose overthrow in an earlier coup in 2006 triggered Thailand's long-running political crisis.

Thaksin is reviled by much of Thailand's Bangkok-based royalist elite, but draws deep loyalty from the poor, but populous northern portion of the country.

His parties have won every election since 2001 - even though the billionaire policeman-turned-businessman fled Thailand in 2008 to avoid jail for a corruption conviction.

Rights groups have condemned the junta for smothering dissent since seizing power.

Protests are banned under martial law while hundreds of activists and political opponents were summoned by the military after the coup - although most were released within days.

The United Nation's last week also expressed alarm at a number of arrests and stiff jail sentences meted out since the coup for defaming Thailand's monarchy.

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