PORTLAND — The success of Apple’s iPad isn’t just drawing more competition to the tablet market. It’s attracting thousands of counterfeit and knockoff products.

On a single day in July, almost 18,000 fakes and clones resembling the iPad and Android devices were available for sale on 23 e-commerce sites, according to MarkMonitor Inc., a San Francisco-based firm that helps companies protect their brands.

The tablets can be illegal — for instance, if they have a bogus Apple logo — and often they don’t work well and have no warranty protection, said Fred Felman, chief marketing officer of MarkMonitor. The copycat products and suspected counterfeits found in MarkMonitor’s survey were offered by more than 5,000 sellers, many of them located in China.

Knockoff iPads may proliferate during the year-end holiday season, as shoppers beset by the economic slump go hunting for bargains. That’s creating more competition for Apple, even if many consumers only buy the tablets because they believe they’re getting the real thing. Apple’s advantage is its software is hard to replicate, said Francis Sideco, an analyst at research firm IHS Inc.

“You can only copy to a certain degree,” he said. For instance, knockoff tablets may not connect to Apple’s iTunes and App Store. “It’s not necessarily about hardware but the software, and it’s very difficult to copy that,” Sideco said.

Trudy Muller, an Apple spokeswoman, declined to comment.