Officials opened two more runways Saturday at Denver's airport after a two-day blizzard as airlines struggled to move thousands of holiday travelers stranded across the country.

Some people had given up hope of spending Christmas with loved ones, however, as airlines were running at near-full capacity with little room for the people whose earlier flights were canceled.

Click here to see FOXNews.com's Colorado snowstorm photo essay

The busiest carrier at Denver International, United Airlines, planned to operate a full schedule of 900 departures and arrivals Saturday for the first time since the storm blew in Wednesday, spokeswoman Robin Urbanski said. Flights were running "close to on schedule," she said.

The airport, the nation's fifth-busiest, expected to have all six runways cleared by Sunday, but there was no telling when the backlog of passengers would be cleared out.

"We don't know," Urbanski said.

The jam in Denver backed up flights around the country heading into one of the busiest travel times of the year, with 9 million Americans planning to take to the air during the nine-day Christmas-to-New Year's period. An estimated 64.9 million people will travel more than 50 miles from home by air, rail and road during the holidays, according to AAA.

In addition, flights were delayed by low visibility Friday in Atlanta and by wind in Philadelphia.

Overseas, fog had grounded flights for most of the week at London's Heathrow Airport, stalling tens of thousands of people who had planned on taking flights at Europe's busiest airport. The fog finally started to lift on Saturday, and British Airways pledged to operate 95 percent of its scheduled flights.

In South America, flight cancellations and hours-long delays caused by overbooking and equipment problems had haunted Christmas travelers across Brazil since Tuesday. Even the Brazilian air force was called in to help move passengers with its fleet of eight passenger jets.

At Chicago's O'Hare International Airport, Army Spc. Nicholas Silva, of Aurora, curled up on a bench Friday for a third night in a terminal. He said he just hoped he could board a plane for home Saturday evening.

"I've slept in worse areas so this doesn't bug me all that much," said Silva, who spent last Christmas stationed in Iraq and was heading home for the first time in two years. "I'll be home for Christmas. I can see my family. Does it really matter after that?"

Denver International closed to all flights Wednesday when the blizzard buried the city in 2 feet of snow, closing schools, offices and stores at the height of the Christmas rush. More than 3,000 incoming flights alone were canceled or diverted during the 45-hour shutdown.

Click here to see FOXNews.com's Colorado snowstorm photo essay

An estimated 4,700 travelers camped out at the airport that night, and close to 2,000 spent a second night on the hard floors and a few cots, hoping to get a place at the front of long lines at ticket counters.

Airport officials did not have an estimate of how many people were still there Friday night.

As planes began taking off again at noon Friday, passengers with long-standing reservations filled most of the outbound flights. That was bad news for those waiting to rebook flights canceled during the storm.

Airline officials tried to explain to unhappy travelers at the airport that they cannot simply bring in extra planes to clear the backlog, and those stranded learned it could be Christmas — or later — before they can catch a plane at DIA.

"When we get an airplane, we run it 10 hours a day every day," Frontier spokesman Joe Hodas said. "It's not like we can decide Dayton's not important and just pull some planes from there."