MADRID -- Spanish Prime Minister Pedro Sanchez said on Tuesday that a power outage which left large parts of Spain and Portugal without electricity for several hours "cannot be allowed to happen again."

Noting that technicians were still investigating the cause of the blackout, Sanchez confirmed that electricity demand had been at normal levels when the outage began at 12:33 local time on Monday.

He also dismissed speculation that an excess of electricity generated by renewable sources was to blame, noting that the disruption had also impacted power generated by Spain's nuclear plants.

Preliminary findings from Spanish grid operator REE suggest that a cyberattack is unlikely to have caused the outage. Nevertheless, Sanchez revealed that national cybersecurity authorities are investigating all possibilities.

"What happened yesterday cannot happen again," Sanchez stressed. "This event must not be repeated. We understand the media have questions, we have questions too, and we will answer them all."

He added that the findings of the ongoing investigation would be used to bolster the security and resilience of Spain's electricity grid.

With nearly 100 percent of the electricity supply restored, Sanchez said Spain was returning to normal. However, he noted that the rail network remained the most affected sector.

Monday's blackout left around 35,000 passengers stranded on Spain's long- and medium-distance trains. Military personnel were deployed to assist in evacuations.

On Tuesday, major train stations in Madrid, Seville, Barcelona, and Valencia faced significant delays due to a backlog of passengers and the displacement of trains, many of which had been immobilized during the outage.