"What can be more significant than the fact that the Maoists have agreed to lock up their arms and are waiting outside (parliament)?" a visibly moved Prime Minister Girija Prasad Koirala said before tabling the new constitution in the House of Representatives (HoR), the powerful lower house of Nepal's parliament.

Despite the opposition parties criticising the constitution earlier during the day, it was passed unanimously with all the 185 MPs present in the house signing the new statute.

As the new constitution was approved after a marathon 11-hour session, Speaker Subhash Nembang dissolved both the HoR and National Assembly, the upper chamber.

Though Maoist chief Prachanda and his deputy Baburam Bhattarai stayed away from the parliament grounds, the rebels led by Krishna Bahadur Mahara marched inside the closely guarded compound.

After 15 years, during which they had been declared terrorists and remained underground, the Maoists were given an office for their parliamentary party, next to that of deposed prime minister Sher Bahadur Deuba's Nepali Congress (Democratic) party.

Home Minister Krishna Prasad Sitaula said the first session of the new parliament will begin soon.

The new unicameral house will have 330 MPs, including 73 Maoist MPs and 10 more supported by them.

This is Nepal's sixth statute since the first promulgated in 1947 by the then all-powerful prime minister Padma Shumsher Rana.

Nembang said the HoR, which was reinstated last year on the crest of a public rising against King Gyanendra's rule, had served the people by asking the government to begin peace talks with the Maoists and remove the terrorist tag on them.

Without naming King Gyanendra, who had seized total power in 2005, he said the house had also fought dictatorship by stripping it of all support - referring to the various measures taken to remove the royal family's powers and privileges, and removing their legal and tax immunity.

With the new constitution, the king loses his position as ceremonial head of state, which now goes to the prime minister.

With the installing of the new constitution and a new unicameral parliament that will include the Maoists, the stage is set for a decisive election in summer when Nepal will decide the fate of its 238-year-old monarchy.

Koirala, battling breathing troubles, promised the new constitution would protect democracy.

"Have no fear," the octogenarian leader said. "The new constitution has some faults but they will be corrected. You have the assurance of a man who has fought for democracy for 60 years."