Groups of people, some carrying signs saying "Justice", walked around the centre of Spain's third-largest city before the demonstration.

The protest comes nearly one year since torrential rain on October 29, 2024 caused flooding in towns near Valencia, killing 229 people, Spain's deadliest natural disaster in decades.

Thousands of people are expected to take part in the protest called by social, civic and labour organisations.

It is set to end near the headquarters of the regional government of Valencia.

Regional leader Carlos Mazon is under immense pressure after his administration failed to issue flood alerts to people's cellphones until hours after the flooding started.

He has has defended his handling of the crisis, saying its magnitude was unforeseeable and that his administration did not receive sufficient warning from central authorities.

Rosa Alvarez, who heads an association representing victims of the floods, blames the regional government's slow response for her 80-year-old father's death.

By the time it issued the mobile phone alert, he was already drowning after floodwaters knocked down one of the walls of his home in Catarroja, she said.

"Every minute counted that day. When the alarm sounded people had already drowned or were in real danger," the 51-year-old social worker told AFP ahead of the march, adding "all those deaths were completely preventable".

Campaigners have staged regular demonstrations against Mazon on or near the monthly anniversaries of the disaster.

The majority of Valencia residents -- 71 percent -- feel Mazon should resign, a poll published earlier this month in daily newspaper El Pais showed.

Mazon is a member of the conservative Popular Party, which sits in opposition to the Socialist-led national government

A state memorial ceremony is due to take place on the first anniversary of the tragedy on Wednesday in Valencia, with King Felipe and Prime Minister Pedro Sanchez due to attend.