Grieving relatives mourned on Sunday the thousands killed in a huge earthquake in Nepal last year, as aid agencies warned about the health risks for the millions still living in sub-standard temporary shelter following the country's worst-ever disaster.

The 7.8-magnitude quake struck high in the Himalayas minutes before noon on a sunny Saturday, toppling one million houses, upending roads and turning hundreds of mountain villages into ruins that left about 9,000 people dead and 22,000 injured.

In Kathmandu, the capital, Prime Minister K. P. Oli led a day of mourning, placing a wreath at the remains of the Dharahara tower that collapsed during the quake, killing 132 people.

At the nearby Durbar Square, a UNESCO-listed world heritage site, a dozen crimson-robed monks chanted from Buddhist scriptures at a memorial as the victims' relatives sat cross-legged, praying, in front of framed portraits of their loved ones.

Among them was grief-stricken Surya Bahadur Shrestha, praying for his late father who was crushed to death by a building in the city.

"I came to mourn my father who died here last year. I prayed for eternal peace for the soul," the 49-year-old said.

One year on from the quake, reconstruction has been slow and uneven in the poor Himalayan country, and most of the $4.1 billion that donors pledged for reconstruction in June last year remains unspent because of political squabbling.

As around 100 people protested near the prime minister's office, demanding the government begin rebuilding, the Red Cross said four million people were still living in poor-quality temporary shelters, posing a threat to their health and wellbeing.

"We are hoping that the government's priorities and perspectives on reconstruction will soon be clear so that we can help people to rebuild and get their lives back on track as quickly as possible," said Max Santner, head of the Nepal mission of the International Federation of Red Cross and Red Crescent Societies.

Gopal Khanal, an aide to Prime Minister Oli, said that the government would make arrangements for proper shelter before the start of the annual rainy season in June, but many are sceptical about the state's ability to deliver.

Kanchhi Tamang, who lost her home in the tremor, said she feared that her three young children would be forced to endure a second monsoon living in a shack on the outskirts of Kathmandu.

A government authority set up in January to oversee rebuilding has so far only distributed $500 to 800 people, against a promise of handing $2,000 to every household who lost their home, National Reconstruction Authority spokesman Ram Prasad Thapaliya said.

Unrest over a new constitution adopted in September, which triggered a months-long blockade of Nepal's border with India by an ethnic group in the south of the country, has added to the upheaval and delays in rebuilding.