WASHINGTON (Reuters) - It was not your average Girl Scout campout.

There were tents, but instead of the backwoods, they were pitched on the South Lawn of the most famous address in America, the White House.

There was star-gazing. But it was led by United States astronaut Cady Coleman, who spent two years on the International Space Station.

And there was a campfire sing-a-long - which President Barack Obama dropped by to join. "What are you guys doing in my yard?" the President said, smiling, as 50 fourth-grade girls bedecked in green badge-covered vests giggled and shouted their hellos. "We're camping on the lawn!" one scout shouted. "We're making history!"

The event was the first-ever Girl Scout campout designed to spotlight first lady Michelle Obama's "Let's Move! Outside" initiative. The First lady, who is also the Honorary National President of the Girl Scouts, sat beside the President on a bale of hay as the group clapped and sang lyrics from souvenir pamphlets.

The Obamas read over the shoulders of the scouts beside them, singing along. "Did you guys see Michelle rockin' out a little bit?" Mr Obama asked at the end of the second song, "I Love Being a Girl Scout".

Later, he turned to face the makeshift campfire, a pile of lanterns arranged in the middle of the circle. "Remember to put the fire out before you go to bed," he said. "That's what Smokey the Bear said."

For the scouts, fourth graders from five troops around the country, the night was an opportunity to learn to rock climb, to pitch a tent, to tie a knot, to look through a telescope - and to get to know Mr Obama. "I thought he was like a serious man who only wanted to work and do business, but he actually has a really nice soft side," said Blaire Batista, 11, from Washington, D.C.

"Until I die...I'm going to have this memory not in the back of my head, but right in the front," she said.

After two songs, Mr Obama turned to the girls to say goodbye.

"Unfortunately, I've gotta go back to work," he said. "But we can have a group hug."

The girls rushed towards him, wrapping their arms around his waist, each one clamoring for a chance to embrace the President.

Outside the circle, the Secret Service stood with their hands clasped, looking around at each other.

Then they started to laugh.