LAS VEGAS (AP):

Katy Perry's doing her best get-out-the vote effort: At a rally for President Barack Obama, she wore a tight white dress imprinted like a ballot, and a square box on her right hip filled in the names of Obama and Joe Biden.

Perry gave a free concert at a park in a historically minority neighbourhood just northwest of downtown Las Vegas to screaming fans at about 9 p.m., the same time Air Force One landed at McCarran International Airport across town.

Obama later told the crowd, "I believe in you. I need you to keep believing in me."

The Las Vegas campaign event drew more than 10,000 people, according to fire officials and organisers, with long lines still on sidewalks during Perry's 30-minute performance before Obama arrived.

The singer opened with a rendition of Al Green's soul hit, Let's Stay Together, and played five songs, including Teenage Dream, before ending with a thumping bass drum version of Firework.

Perry, who recently also played a free concert at an Obama event in Los Angeles, paused before the last song on Wednesday to exhort people in the Las Vegas crowd to vote early.

"Don't wait. Go tomorrow," she said. "How many of you are 18 here? It's going to be your first time, right?"