HUA HIN, May 5 - Thailand's revered King Bhumibol Adulyadej made a rare public appearance Monday to mark the 64th anniversary of his coronation, as the political turmoil gripping his kingdom enters a critical phase.

The king's appearance comes as embattled prime minister Yingluck Shinawatra faces two key legal challenges which could see her removed from office over the coming days, while opposition leader Abhisit Vejjajiva refused at the weekend to commit to elections mooted for July to end the political crisis.

King Bhumibol, the world's longest-reigning monarch and a father figure for Thais, was crowned on May 5 1950, although he came to the throne in June 1946 following the death of his elder brother.

The 86-year-old monarch is seen as a moral authority in Thailand, which has been deeply divided along political lines since 2006 when billionaire former premier Thaksin Shinawatra -- Yingluck's older brother -- was ousted in a military coup.

King Bhumibol's public speeches are closely scrutinised especially in times of political crisis, but on this occasion he did not speak.

The streets near his coastal palace were a sea of yellow as thousands of people, wearing the king's signature colour, waved flags and shouted "long live the king" as the monarch's vehicle passed through the central coastal town of Hua Hin, where he has lived since leaving a Bangkok hospital last August.

A short service was held in a room in the royal palace packed with Thailand's political and military establishment as well as senior members of the royal family including the heir Crown Prince Maha Vajiralongkorn.

Monks led prayers as the king, who has suffered ill health for several years and uses a wheelchair, looked on.

Yingluck was also present. She has faced six months of protests demanding she steps down, which have left at least 25 people dead and hundreds more wounded, raising fears of wider violence between pro- and anti-government supporters as legal moves against her edge towards a conclusion.

Crunch time

Yingluck is due to appear before judges at the Constitutional Court on Tuesday against an allegation of abuse of power over the transfer of a top security official.

"This is a critical few weeks for the future of Thailand," Noppodon Pattama, Thaksin's legal adviser, told AFP.

"If the (constitutional) court judgment is fair it could unblock the political conflict... If it is not fair, it will make things worse."

The premier could also be charged with neglect of duty by Thai anti-graft officials over a bungled and costly rice subsidy policy which could see her toppled and banned from politics.

It is not clear when either ruling will be made, but they are expected over the next 10 days.

Supporters of both sides have vowed major rallies in step with the verdicts, stoking concerns over clashes in a kingdom whose recent political history has been pock-marked by violence.

Thailand's political schism roughly pits the Bangkok elite and middle class -- as well as staunchly royalist southerners -- against the Shinawatra's rural electoral base in the northern portion of the country and many among the urban poor.

Abhisit, the leader of the opposition Democrat Party, last week called for Yingluck to resign to make way for an appointed interim premier who would oversee a referendum on reforms.

He said elections could then be held, but refused to back a July 20 date already agreed by Yingluck's current caretaker government and poll authorities.

His 'roadmap' has been given short shrift by ruling party officials, while anti-government protesters camped out across Bangkok are yet to publically endorse his proposal.

"I think Abhisit is trying to save his political career," said Paul Chambers, director of research at the Institute of South East Asian Affairs at Chiang Mai University, warning his party may "fall apart" if it does not win round the protesters.

Thaksin, a billionaire tycoon-turned-politician, fled overseas in 2008 to avoid jail for a corruption conviction, but he is seen as pulling the strings behind his sister's premiership and is accused by opponents of nepotism and rampant corruption.

But he is a hero to his mainly rural, poor supporters for recognising their burgeoning political and economic aspirations.

Shinawatra led or allied government have won every election since 2001.