WASHINGTON (Reuters) - US President Barack Obama and his family led the lighting of the National Christmas Tree on Thursday, with charity, thanks, and a moment of reflection for the victims of Wednesday's shooting.

"Five, four, three, two, one. Merry Christmas, everybody!" Obama said as the national tree took sparkle for the 93rd year.

Flanked by his wife, two daughters and mother-in-law, Obama led the countdown in windy President's Park, across the street from their home at the White House, in a celebration that included performances by pop singer Aloe Blacc and rock band Fall Out Boy.

Using a moment in the festive holiday setting to address the victims of Wednesday's deadly shooting spree in California, Obama struck a more somber tone, cautioning Americans to "summon the spirit of togetherness" this holiday season.

"Now, this is, of course, the most wonderful time of the year. But we would be remiss not to take a moment to remember our fellow Americans whose hearts are heavy tonight - who grieve for loved ones, especially in San Bernardino, California," Obama said in reference to the rampage that left 14 dead and 21 wounded.

"Their loss is our loss, too, for we're all one American family. We look out for each other in good times, and in bad. And they should know that all of us care about them this holiday season," he said.