Nepal's prime minister joined Buddhist monks at a prayer ceremony Monday to mark the start of rebuilding of five ancient monuments destroyed in an earthquake that killed thousands and devastated the country's rich cultural heritage.

The ceremony at the seventh-century Swayambhunath Temple complex came exactly a year after the quake struck, although the main commemorations for the dead were held Sunday, the anniversary by the Nepali calendar.

One of the Buddhist temples at the spectacular hilltop complex, a UNESCO world heritage site, was completely destroyed by the 7.8-magnitude quake.

Rebuilding work began Monday on that and four other monuments, including a temple in the historic town of Bhaktapur and two wooden pavilions once used for royal ceremonies that crumbled in the quake.

"They are treasures given to us by our ancestors... it is our responsibility to hand them down to the next generation the way they were handed down to us," said Bhesh Narayan Dahal, director general of the government's Department of Archaeology.

"We will begin reconstruction of other monuments as well, work has been slow but now it will pick up pace," Dahal told AFP.

About 700 monuments require rebuilding or repair. Many are major tourist draws, including Swayambhunath, known as the "Monkey Temple" because of the animals occupying the steps leading up to it.

While restoration work has begun on a handful of temples, including the fifth-century Changu Narayan complex, officials say it will be years before Nepal's rich architectural heritage can be fully restored.

The rebuilding of houses has been even slower to start, and on Sunday protesters marched on government offices to demand faster reconstruction efforts.

The Red Cross says four million victims are still living in flimsy shelters after the disaster, which killed nearly 9,000 people, including hundreds of Nepalis and tourists who died in a massive quake-triggered avalanche that destroyed the entire village of Langtang.

- 'I felt heartbroken' -

Dozens of foreign trekkers travelled to Langtang Monday for a memorial service and paid tearful tribute to victims, many of whose bodies were buried so deep under debris that they have never been recovered.

Buddhist monks led the service that was attended by around 100 people, including villagers who have returned to Langtang to rebuild their lives.

"Someone said I lost my mother, another said I lost my daughter or son or wife or father... Some people spoke, others couldn't speak because they were crying," said guesthouse owner Palsang Tamang, who also took part in the ceremony.

Survivors unveiled a marble wall inscribed with the names of those killed, including Tamang's daughter.

"I felt heartbroken, I lost my daughter and so many other family members. We prayed for them. We hope they all find peace and are in heaven," he told AFP.

On Everest, mountaineering teams caught up in the tragedy observed a minute's silence at 11:56 am -- the time the earthquake hit -- in memory of their fallen colleagues.

The mountain suffered its deadliest ever disaster when the quake triggered an avalanche that killed 18 people.

"This was an opportunity to remember those who died, those who were injured and the many people who worked so hard to rescue and treat the 100 patients," Rachel Tullet, base camp manager for Jagged Globe, said in a blog post.

Nepal has issued 289 permits to mountaineers for the brief spring climbing season, which lasts from mid-April to May, and many have begun to ascend the world's tallest peak.

"Tourism has suffered after the quake, but we did not see much impact (on Everest)," said tourism chief Sudarshan Prasad Dhakal.

A spokesman for Nepal's National Reconstruction Authority (NRA), which is overseeing rebuilding, told AFP that work would soon start on schools and hospitals across quake-hit districts.

The disaster damaged about 8,000 schools and 1,200 health centres and Nepal's government has come under fire for doing little to rebuild classrooms or rural medical facilities that are a lifeline for remote communities in hilly regions.