King Felipe VI will lead mourners at the ceremony, which is set to get underway at 6:00 pm (1700 GMT) in Valencia, Spain's third-largest city on the Mediterranean coast.

Prime Minister Pedro Sanchez and regional leader Carlos Mazon -- who is under fire over his response to Spain's deadliest floods in a generation -- are also expected to attend.

Joining them will be mayors from the 78 municipalities hit by the floods, mostly in the southern outskirts of Valencia, and around 800 relatives of the victims.

The event will take place at a museum in the City of Arts and Sciences, a cultural and architectural complex surrounded by shallow pools in Valencia.

The regional government has declared a day of mourning, while the town of Paiporta, at the epicentre of the disaster, will observe three days of remembrance.

"Any slightly cloudy day, you can sense that we're not okay, because we are a traumatised society," Marilo Gradoli, the head of an association representing victims of the floods, told AFP.

In last year's natural disaster, torrential rain unleashed flooding that killed 229 people in towns near Valencia.

Seven more people died in the neighbouring Castilla-La Mancha region, and one person died in Andalusia in the south.

The deluge swept away 130,000 vehicles and damaged thousands of homes, generating 800,000 tonnes of debris.

- 'On our own' -

Mazon's regional administration has been heavily criticised for not sending out alerts to mobile phones until 8:11 pm -- when flooding had already started in some places.

That was more than 12 hours after the national weather agency had issued its highest alert level for torrential rains.

Despite signs of severe flooding, Mazon went ahead with an hours-long lunch with a journalist on the day of the catastrophe.

He has defended his handling of the crisis, saying its magnitude was unforeseeable and that central authorities did not provide sufficient warning about the severity of the rain.

Anger as well as sadness remain vivid among residents of the affected areas.

"We were really on our own," said Doly Murcia, 50, from Paiporta, where 56 people died and furious survivors hurled mud at the visiting monarchs and Sanchez in the immediate aftermath.

More than 50,000 people took to the streets of Valencia city on Saturday to demand that Mazon resign over his response to the floods, the latest in a string of such demonstrations.

Mazon has frequently been heckled when appearing at public events, and some victims' families have called on him to stay away from the state memorial.

But his conservative Popular Party, which sits in opposition to the Socialist Sanchez at the national level, has insisted he should be present as the representative of the Valencian people.

A judicial investigation into the emergency response is underway.

Under Spain's decentralised system, managing disasters falls under the authority of regional governments.