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Katy Perry, Matt Dallas

AP Photo/Chris Pizzello

Katy Perry was the belle of ball at the Gridlock New Year’s Eve party in Hollywood.

This year’s pop princess newcomer performed for about 5,000 people on the Paramount lot.

After leading the countdown to '09, she shouted, "Happy f--king New Year, everyone!" and continued into a performance of her latest hit, "Hot N Cold." She then dedicated her tune, “Waking Up in Vegas,” to—who else?—partiers in Sin City.

Of course she sang “I Kissed a Girl.” “Has everyone kissed someone yet?” she asked, before launching into her debut hit single. 

Perry wasn't the only happening in Hollywood...

Usher, Tameka Foster

The Bank Nightclub

Across town, at the Wiltern, DJ AM and Travis Barker performed together for the first time since their tragic plane crash in September. The two headlined the New Year’s Nation celebration, which was broadcast live to several cities around the country and streamed live on the Internet.

Over in Vegas, Usher hosted the bash at Bank nightclub at the Bellagio. It was the first night out for him and his wife, Tameka, since the arrival of their second child.

Shortly after midnight, Usher performed  “Love in This Club," “Bad Girls” and “Yeah.”

Before hitting the club, Usher helped Tameka celebrate her birthday with a dinner for 25 at Fix restaurant & Bar. As for the cake? Decorated as two Louis Vuitton trunks.

Also in Vegas, Fergie gave a concert at the Venetian Ballroom before heading to Lavo at the Palazzo Hotel to host the night’s festivities. She and hottie fiancé Josh Duhamel, who will wed on January 10, locked lips as the new year began.

In Miami, Gerard Butler and Maria Menounos led the countdown at the Shore Club’s Skybar. New York nightlife gurus Eugene Remm and Mark Birnbaum hosted the party, which attracted Twilight’s Kellan Lutz, AnnaLynne McCord, Jared Leto and Kristin Cavallari.

—Additional reporting by Dahvi Shira