MADRID, April 29 (Reuters) - Spain's electricity grid

operator Red Electrica said it was able to supply

virtually all of the country's electricity demand early on

Tuesday as the system gradually recovers from a nationwide

blackout on Monday, although most trains were still not running.

All of Spain's substations were operating on Tuesday

morning, Red Electrica said in a post on X social media. "We

keep on working from centre of electric control to secure total

normalization of the system," it added.

The Madrid underground metro network said it had resumed

operating at 8 am (0600 GMT) with 80% of trains circulating, but

railway infrastructure operator Adif said most trains nationwide

were not operating.

A huge power outage hit most of the Iberian Peninsula on

Monday morning, bringing both Spain and Portugal to a standstill

- grounding planes, halting public transport and forcing

hospitals to restrict routine proceedings.

Power supply was gradually restored in both countries from

late Monday afternoon and early evening though some operations

were still not able to resume on Tuesday morning.

The cause of Monday's power outage was unclear.

Prime Minister Pedro Sanchez said that the country had

suffered a loss of 15GW of electricity generation in five

seconds, equivalent to 60% of national demand.

The loss triggered a disconnection of the Spanish and French

grids, prompting a general collapse of the Spanish system, Red

Electrica's chief of operations Eduardo Prieto told reporters on

Monday evening. Some areas in France suffered brief outages on

Monday.

Portugal's grid officials suggested the issue originated in

Spain.

Spain is one of Europe's biggest users of renewable energy

sources, but Monday's shutdown has already sparked debate about

whether the volatility of supply from solar or wind has made its

power systems more vulnerable to such an outage.

The reasons for the loss of power are unknown, Sanchez said,

adding no hypotheses are ruled out, he added.

Portuguese Prime Minister Luis Montenegro said there was "no

indication" a cyberattack was behind the outage.

Spain's Sanchez spoke to NATO Secretary General Mark Rutte

on Monday, NATO sources said in Brussels.