The cause of the massive power outage that wreaked havoc in Spain and Portugal remains a mystery Tuesday as service was restored to the Iberian Peninsula. 

By 7 a.m. local time, more than 99% of energy demand in Spain had been restored, the country's electricity operator Red Eléctrica said. Portuguese grid operator REN said Tuesday morning all the 89 power substations had been back online since late last night and power had been restored to all 6.4 million customers. 

"We have never had a complete collapse of the system," Spanish Prime Minister Pedro Sánchez said in a televised address Monday night.  

Emergency workers in Spain said they had rescued some 35,000 passengers on Monday who were stranded along railways and in underground tunnels. 

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Video that aired on Spanish television showed people evacuating metro stations in Madrid, and empty stations with trains stopped in Barcelona. Spain’s parliament was also left in the dark, public broadcaster RTVE reported.  

The ATP Tour said play at the Madrid Open tennis tournament was suspended for the day due to the power outage.  

In Portugal, several Lisbon subway cars were evacuated, courts stopped working and ATMs and electronic payment systems were affected. Traffic lights in Lisbon also stopped working during the outage. 

REN, Portugal's grid operator, described the incident Monday as a "rare atmospheric phenomenon." 

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"Due to extreme temperature variations in the interior of Spain, there were anomalous oscillations in the very high-voltage lines, a phenomenon known as induced atmospheric vibration," it was quoted as saying. "These oscillations caused synchronization failures between the electrical systems, leading to successive disturbances across the interconnected European network." 

However, on Tuesday, Spain’s meteorological agency AEMET said that it had not detected any "unusual meteorological or atmospheric phenomena" Monday and no sudden temperature fluctuations were recorded at their weather stations. 

Eduardo Prieto, Red Eléctrica’s chief of operations, said the instability in the power grid caused the Spanish and French electricity interconnection through the Pyrenees mountains to split, leading to a failure on the Spanish side, according to Reuters. The news agency reported that some parts of France suffered brief power outages on Monday as well. 

Authorities were still investigating what happened on Tuesday. 

The Associated Press contributed to this report.