Hours after BRITNEY SPEARS tied the knot in Las Vegas over the weekend, two eyewitnesses came exclusively to Entertainment Tonight to tell us what they saw inside the wedding chapel.

The couple, who asked to remain anonymous, described the pop princess as "nervous," and her husband-to-be, JASON ALLEN ALEXANDER, as "a bit blank."

"When they walked in, Britney was wearing a white, baseball cap, had her hair tucked under it, kinda messy, pieces falling out," the witness told us. "My wife noticed that she was nervous. And I noticed too. She was looking at the order form for the wedding packages. You could see it trembling in her hands. When I looked at her, her hand was shaking with the paper, and you could see the paper kind of fluttering. He didn't seem nervous. He seemed a bit blank."

GEORGE MALOOF JR., who owns and operates the hip Palms Casino Hotel in Vegas, confirmed that the pop star tied the knot at about 5:30 Saturday morning at the Little White Wedding Chapel on the Vegas Strip. However, by Saturday afternoon, the wedded bliss seemed to be over.

Now, she's seeking an annulment and her brand-new husband, a childhood sweetheart, is already back home in Kentwood, LA.

A spokesperson for the pop princess released this statement to ET Sunday: "On January 3, Britney Spears and a friend took a joke too far by getting married. Ms. Spears and Mr. Alexander have filed for an annulment, which will become official on Monday, January 5th."

However, our sources said the couple seemed quite serious: "I've never seen a less funny joke. If it were a joke, they should have probably been smiling and enjoying it more."

ET also spoke to JASON ALLEN ALEXANDER's grandfather, ROBERT ALEXANDER, who said of his grandson's return, "He'd been through a lot. We picked him up in New Orleans coming back and he didn't have much to say." The elder Alexander also revealed that Britney and her husband Jason had spent time together recently, including on Christmas Eve. Of the marriage, Robert said, "I wish them well. Whatever happens, happens."

The bride wore torn jeans, a black top and a baseball cap to the impromptu nuptials, our eyewitnesses said. She walked down the aisle escorted by a Palms' bellman and was sporting a garter belt around her thigh after the ceremony.

Watch ET Monday as we count down the events leading up to her walk to the altar and show you an exclusive wedding photo of Britney taken by our eyewitnesses. Click here to see the marriage license.