The king of Thailand has officially endorsed the army chief and coup leader Gen Prayuth Chan-ocha as prime minister, three months after the military leader took control of the nation in a bloodless coup.

The approval on Monday – a mere formality from the ageing King Bhumibol Adulyadej – follows Prayuth's appointment last week by the military-majority national assembly, who voted in the sole candidate unanimously.

Dressed in a while military uniform and flanked by officers, Prayuth said: "I consider this the highest honour of my life," and added: "I am ready to get tired."

The royal endorsement will allow Prayuth – who is due to stand down as army chief next month – to establish an interim government until elections are held some time in late 2015. He is expected to form a new cabinet by October and described his priorities as preparing the country for national reform and establishing prosperity, according to media reports.

"Our country has accumulated many problems … which need to be urgently solved," he said. "To do this we must not create future problems."

Prayuth, 60, is the first coup leader to serve as prime minister in nearly 60 years and his appointment was condemned by opponents.

The ruling junta, named the National Council for Peace and Order (NCPO), has also come under fire both at home and abroad for cracking down on dissidents; detaining politicians, journalists, critics and activists; shutting down newspapers, radio and TV stations; imposing martial law; and handpicking a military-dominated parliament that now has more officers in it than Burma's.

In a statement, the overseas-based Organisation of Free Thais for Human Rights and Democracy described Prayuth's appointment as "a political farce and in violation of the rule of law". Prayuth himself has promised a "Thai-style" democracy and has staged various "happiness festivals" around the capital Bangkok in order to "bring happiness back to the Thai people".

Prayuth seized control on 22 May after six months of sometimes bloody protests that left the nation in legislative paralysis and saw 28 people killed and over 700 injured.

The coup removed the democratically elected PM, Yingluck Shinawatra, from office eight years after her brother Thaksin was also removed from his post as prime minister – in yet another coup that also involved Prayuth.