Power in Spain and Portugal was almost fully restored by early Tuesday morning after a sweeping blackout hit the two countries and parts of France on Monday noon.

Red Electrica, Spain's power operator, said as of 6 am, 99.16 percent of power has been restored to mainland Spain.

"All substations on the transmission grid are operational. We continue with the restoration work," the operator said.

Metro services in Madrid resumed operations at 8 am, except for Line 7A, according to Isabel Diaz Ayuso, president of the Community of Madrid. She said 80 percent of trains will "run during a normal morning rush hour" and all public city buses and intercity train lines are running as normal and will be free all day.

"The region's six transport interchanges remain open as usual," she said.

In Portugal, about 6.2 million out of the 6.5 million households in the country had electricity back by Monday morning. Energy operator Ren said it has managed to get power to "85 of the 89 substations and switch stations" and hopes to fix the remaining issues on Tuesday.

Portuguese media reported that Lisbon Metro was still down earlier on Tuesday.

The massive power outage brought much of Spain and Portugal to a standstill and caused huge chaos. On the Iberian Peninsula, with about 60 million people, traffic was disrupted and many people were stranded inside metro tunnels. People were unable to pay with their bank cards, use their phones and access the internet. Families were seen sitting at their homes lit by candlelight.

A state of emergency declared in Spain before Monday midnight was lifted on Tuesday afternoon.

The cause of the power outage remains unclear. Portugal's Prime Minister Luis Montenegro said there is "no indication" of a cyberattack. The Portuguese government said the outage appeared to stem from problems outside the country, an official told the national news agency, Lusa.

Spain's Prime Minister Pedro Sanchez warned against speculation. In his address to the nation 11 hours after the country ground to a halt, Sanchez said government experts were still trying to determine what caused the outage.

"We have never had a complete collapse of the system," he said, noting that at 12:33 pm on Monday, Spain's power grid lost 15 gigawatts, the equivalent of 60 percent of its national demand, in a matter of five seconds.

Red Electrica's Head of Operations Eduardo Prieto said the event was "exceptional and extraordinary".

Many Spanish residents went to sleep in total darkness on Monday night. In Madrid, cheers erupted from balconies of homes where electricity returned.

Cellphone services were spotty on Tuesday morning as people were still struggling to call their loved ones.

Spanish media showed images of people walking on the street to get home after metro and train services stopped, and some commuters clambering out of stalled trains in unlit tunnels.

Such a widespread power outage across the Iberian Peninsula was unprecedented. However, Spain's Canary Islands, Balearic Islands and territories of Ceuta and Melilla, located across the Mediterranean in Africa, were not affected.

Agencies contributed to this story.