HAVANA, March 16 (Reuters) -- Cuba's national electric grid collapsed on Monday, the country's grid operator said, leaving around 10 million people without power amid a U.S.-imposed oil blockade that has crippled the island's already obsolete generation system. Grid operator UNE said on social media it is investigating the causes of the blackout, the latest in a series of widespread outages that last for hours or days and that this weekend sparked a rare violent protest in the communist-run country.