"The king and his supporters are trying to become politically active again. We won't tolerate this," Ganesh Man Pun, the leader of the Young Communist League, told a cheering crowd.
Dozens of armed police lined the street leading to the palace where around 1,000 of the king's supporters queued to enter the complex and greet him as birthday celebration got under way.
The king's supporters waited for hours in the scorching sun outside the pink palace to offer the monarch bouquets and gifts. Some beat cymbals, blew copper pipes and chanted "long live the king" and "our king is dearer to us than our hearts".
Revered by supporters as the reincarnation of Vishnu, the Hindu god of protection, King Gyanendra was badly discredited after he sacked the government and assumed total power in 2005.
|"It is almost certain that the king's birthday party is going to be a fiasco" |
Kathmandu Post newspaper editorial
His future as monarch is now in danger after former Maoist rebels and mainstream political parties made peace and formed a government earlier this year.
The new government stripped Gyanendra of almost all his powers including the control over the army and taxed his property and income.
Gyanendra has planned a huge reception on Sunday but student groups have vowed to try to spoil the celebrations. Foreign ambassadors have also snubbed royal invitations to attend the birthday bash.
"It is almost certain that the king's birthday party is going to be a fiasco," the English language Kathmandu Post said in an editorial on Friday.
"The birthday party was suddenly considered as a royal ploy to taste the political clout of the king. Hence, rejection," it said.
The US embassy spokeswoman said the ambassador would not be attending because "we saw no useful purpose".
"The future of the monarchy is for the people of Nepal to decide," Sharon Hudson-Dean said, signalling that Washington - previously seen as being supportive of the king - is no longer willing to stick its neck out for him.
The fate of King Gyanendra and his 238-year-old Shah dynasty is to be decided in the outcome of crucial polls planned for November, when the country will elect a body that will rewrite the constitution.
Girija Prasad Koirala, the elderly prime minister, has said that the king should step down before the elections, but the monarch shows no sign of budging.
King Gyanendra came to the throne in 2001 in tragic and bizarre circumstances after an apparently drunk and drugged Crown Prince Dipendra killed most of his family, including the king and queen, and then himself.