Nepal's former Maoist rebels paid tribute to fallen comrades Saturday in a ceremony marking 20 years since the start of an insurgency that transformed the Himalayan nation from a Hindu monarchy to a secular republic.

On 13 February, 1996, Maoist guerrillas attacked a police post in western Nepal's Rolpa district, launching a decade-long civil war that eventually claimed some 16,000 lives and left hundreds of people missing.

Hundreds of Maoist cadres gathered at the party's office in Kathmandu, waving red flags as senior leaders placed garlands on the "martyr's pillar" -- a monument built to honour fallen and missing combatants.

The rebels laid down arms in 2006 before entering politics and eventually helping to draft the country's new national constitution.

Introduced in September, the charter established Nepal as a secular federal republic, reflecting Maoist ideology.

"The constitution is the product of our war and we... take ownership of the new constitution," Maoist chief Pushpa Kamal Dahal, better known by his nom-de-guerre Prachanda, told cheering cadres in Kathmandu.

But for many ordinary Nepalis, who voted for the party in Nepal's first constituent assembly elections held in 2008, the Maoists failed to deliver on their pledge of bringing equality and progress to the deeply feudal country.

"Many people lost their lives, many went missing or became disabled so things would change in this country," Rina Tamang, a shopkeeper in Kathmandu, told AFP.

"Now we have a new constitution but we are still waiting for the change the Maoists promised us. Personally, I have no hope left anymore," the 39-year-old said.

After sweeping to victory in the 2008 polls, the former rebels soon came under fire for abandoning revolutionary ideals and developing a taste for luxury.

They alienated their voter base and crashed out in Nepal's second constituent assembly elections in 2013, finishing in third place.

"A few leaders compromised on their promises, a few betrayed the revolution for lucrative positions in government... all this needs to be rectified to bring real change," said former guerrilla, Laxmi Prasad Chaulagain.

The constitution, the first drawn up by elected representatives, was meant to bolster Nepal's transformation into a peaceful democratic republic after decades of political instability.

But it has instead sparked violence, with more than 50 people killed in clashes between police and demonstrators from Nepal's Madhesi ethnic minority, who say it leaves them politically marginalised.

Ongoing discussions between the government and protesting parties have failed to yield an agreement.