Police lined the street leading to the palace as supporters queued to enter the sprawling pink stone compound and meet their revered monarch.

"His majesty has made mistakes in the past but his mistakes are not inexcusable. We should maintain the monarchy because it's a symbol of national unity," 27-year-old Ratna Magar said as he waited in line.

Nine people were injured as the two sides clashed in Durbar Square in the ancient heart of Kathmandu just south of the palace, police said. Two officers were among the injured.

"Royal supporters had gathered to attend the king's birthday when the Young Communist League activists attacked," said police officer Surya KC, adding officers broke up the battles after about 20 minutes.

The palace press office said 15,000 people paid their respects to the king, but an AFP reporter who watched visitors file in said there were no more than 2,000.

It was a sharp contrast to the tens of thousands who used to offer the king birthday greetings before he seized direct control of Nepal in 2005 in what he said was a bid to defeat Maoist rebels waging a bloody "people's war."

On Saturday, musicians dressed in traditional costumes, playing drums and long horns, serenaded the monarch outside the palace, as children and orange-robed Hindu holy men waited in sweltering heat to enter.

Saturday's clashes came after the fiercely republican Young Communist League held a rally of around 1,000 members near the palace whose approach road was blocked off by police in a bid to avert trouble.

"The king and his supporters are trying to become politically active again. We won't tolerate this," their leader Ganesh Man Pun told a cheering crowd.

Earlier on Saturday around 200 pro-royalists staged a procession, shouting "long live King Gyanendra" and holding his portrait aloft.