By Susana Vera

VALENCIA, Spain (Reuters) - An arts and science centre which normally plays host to opera performances was on Saturday transformed into the nerve centre for the clean-up operation after catastrophic floods in eastern Spain which have claimed at least 207 lives.

Volunteers went to Valencia's City of Arts and Sciences for the first coordinated clean-up organised by regional authorities.

On Friday, the mass spontaneous arrival of volunteers complicated access for professional emergency workers to some areas, prompting authorities to devise a plan on how and where to deploy them.

Carlos Mazon, Valencian regional president posted on X on Friday: "Tomorrow, Saturday, at 7 in the morning, together with the Volunteer Platform, we will launch the volunteer center to better organize, (and) transport the help of those who are helping from the City of Arts and Sciences in Valencia."

Prime Minister Pedro Sanchez was due to address the nation on Saturday morning.

In some of the worst-hit areas, people have resorted to looting because they have no food or water. Police said on Friday they had arrested 27 people for robbing shops and offices in the Valencia area.

More than 90% of the households in Valencia had regained power on Friday, utility Iberdrola said, though thousands still lacked electricity in cut-off areas that rescuers struggled to reach.

Some 2,000 soldiers were deployed to search for people who are still missing and help survivors of the storm, which triggered a new weather alert in the Balearic Islands, Catalonia and Valencia, where rains are expected to continue during the weekend.

Officials said the death toll is likely to keep rising. It is already Spain's worst flood-related disaster in more than five decades and the deadliest to hit Europe since the 1970s.

(Reporting by Graham Keeley; Editing by Angus MacSwan)