One of the heaviest travel weeks of the year began with the cancellation of hundreds of flights on Sunday at the world\u2019s busiest airport in Atlanta, where a power outage left passengers stranded in darkened terminals or in aircraft idling on tarmacs.\r\nThe early afternoon outage paralyzed operations at the Hartsfield-Jackson Atlanta International Airport through Sunday evening. Even though power was finally restored at close to midnight Sunday, the incident continued to wreak havoc on holiday travel plans for thousands of people hit by airline cancellations extending into Monday.\r\nDelta\u00a0(DAL, +1.47%)\u00a0said it was cancelling about 300 flights on Monday, on top of the 900 Sunday cancellations as a result of the Atlanta outage. United Air Lines\u00a0(UAL, +0.98%)also warned on social media that travel on Monday may be affected.\r\nThe incident led the Federal Aviation Administration to ground flights bound for Atlanta.\u00a0American Airlines\u00a0(AAL, +1.73%)\u00a0and\u00a0Southwest Airlines\u00a0(LUV, +1.30%)\u00a0were also among the major carriers that suspended operations at the airport on Sunday. Southwest canceled 70 departures on Sunday.\r\nAll passengers had safely disembarked from aircraft by approximately 10 p.m., or nine hours after the outage began, Atlanta Mayor Kasim Reed said on Twitter\u00a0(TWTR, -1.62%). On Twitter, some passengers reported sitting on planes for hours.\r\nReed had told reporters earlier in the evening that the airport was expected to be fully operational again by midnight.\r\nThe FAA said air traffic control remained fully staffed, with the airport open and accepting general aviation and cargo operations. It expected commercial traffic to resume on Monday.\r\nReuters.