Officials are complaining that tickets given away for a city-sponsored centennial concert by the bands the Red Hot Chili Peppers and Weezer are being sold on the Internet.
"You get a couple of bums who take advantage of the situation, and what starts off as a very nice gesture ... turns into a sleazy methodology of trying to make a buck from it," Las Vegas Mayor Oscar Goodman said.
The July 2 show in a parking lot near the Las Vegas Convention Center is to mark the city's 100th anniversary.
About 35,000 free tickets were quickly snapped up when Event promoter Clear Channel Entertainment Properties made them available online through a ticket service on Monday. Minutes later, traders began offering tickets through eBay auctions at prices that by Thursday ranged from $25 and $70 per ticket.
"It's so unfair because the centennial committee wanted to present this very special event available to everybody free of charge," Goodman said.
Officials said another 5,000 tickets will be made available through TicketMaster on May 16, while 10,000 were being distributed to hotels.
Clark County and the city of Las Vegas forbid ticket scalping. Goodman said resellers may be prosecuted.