The king formally named Girija Prasad Koirala as Nepal's new prime minister Thursday, although a spokesman said the 84-year-old politician was sick with a lung problem and had been on oxygen throughout the day.

Koirala's unspecified illness kept him away from a victory rally in Katmandu, marring a day that began with the communist rebels announcing a three-month cease-fire.

The temporary truce lifted a key burden on the new government poised to take control after weeks of bloody protests in which 15 people were killed by soldiers and police. The demonstrations forced King Gyanendra to reinstate Parliament.

Koirala was the choice for prime minister by the alliance of political parties behind the protests — which were backed by the Maoist insurgents — and his illness could keep him from Friday's first session of the reinstated Parliament.

Spokesman Krishna Sitaula said Koirala — who is taking his fifth turn as prime minister — would be examined by doctors before deciding whether to go to Parliament.

CountryWatch: Nepal

The outlawed student wing of the rebels, meanwhile, announced they would rally in Katmandu on Friday. The All Nepal National Free Student's Union (Revolutionary) was outlawed by the previous government and declared a terrorist organization.

Earlier Monday, the rebels' elusive leader, Prachanda, said in a statement that his fighters would refrain from any assaults on government targets for three months to give Nepal a chance for peace.

"We declare a unilateral cease-fire for three months through this statement to express deep commitment to people's desire for peace," the statement said.

After Gyanendra announced Monday that he would meet a key demand of the parties and reinstate Parliament, dissolved in 2002, the rebels initially rejected the overture as a ruse by the king to keep his crown.

They warned that the parties' quick acceptance of the deal was a betrayal of previous agreements between the opposition and Maoists. Both want a new constitution to limit the role of the monarchy — or eliminate it altogether.

By Wednesday, however, the Maoists had softened their position, lifting a blockade of highways. Thursday's announcement clearly reflected a willingness by the rebels to give politicians a chance to set up a constitutional convention.

The rebels' cease-fire announcement followed a conversation between Prachanda and Koirala that helped ease misunderstandings between the two groups, the prime minister said earlier in the day.

Still, the Maoists were clear that the cease-fire is for three months only and "with the intention to encourage the political parties to announce an unconditional special assembly."

"Our fight will continue," Matrika Yadav, the highest-ranking Maoist leader imprisoned by the government, said in an interview Wednesday, adding that the group's eventual goal remains nothing short of creating a communist state.

The rebels are for now "being flexible in order to trust the parties" to help overthrow the king, Yadav said. "If anyone goes against the people, their downfall is inevitable, whether it is the political parties or the monarchy or any force."

The rebels have declared cease-fires three times, but none resulted in any significant progress toward a permanent peace.

The rebel campaign has left nearly 13,000 dead in a decade, and crushing the insurgents was one of the top reasons Gyanendra gave when he seized power in February 2005, dismissing an interim government.

The parties welcomed the rebel cease-fire.

"This will help bring the Maoists to the negotiating table for peace talks that could end the violent conflict," said Gopalman Shrestha of the Nepali Congress Democratic Party.

Tens of thousands of people gathered Thursday for a victory rally in central Katmandu to hear party leaders speak.

Waving party flags — such as the red and white stripes of the Nepali Congress and the hammer and sickle of the Communist Party of Nepal — crowds poured into a park. Some were singing and chanting as they marched down Katmandu's narrow lanes.

As the rally began, the crowd cheerfully demanded to hear from Koirala, and angrily threw rocks, plastic bottles and wooden sticks at the speakers when they heard he was not well and at home resting, briefly disrupting the rally.

But politicians quickly calmed the crowd by offering assurances that Koirala was genuinely unwell.