Pro-royalists plan a march to show their support for the monarch on Saturday, and the palace has asked the government to provide increased security over the weekend.
Revered by supporters as the reincarnation of Vishnu, the Hindu god of protection, King Gyanendra's future as monarch is now in grave danger after former Maoist rebels and mainstream political parties made peace and formed a government earlier this year.
King Gyanendra, badly discredited after he assumed and tried to hold on to dictatorial powers to beat back the Maoists, plans to throw open his palace in Kathmandu for well-wishers on Saturday and stage a reception on Sunday.
But protests have already started, and on Friday afternoon around 400 students blocked traffic in central Kathmandu and held a noisy rally calling for people to ignore the celebrations.
Ambassadors from countries including India, the United States and the European Union will be staying away.
"The birthday party was suddenly considered as a royal ploy to taste the political clout of the king. Hence, rejection," the Kathmandu Post editorial 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.