HTC was the first company to manufacture a cell phone based on Google Inc.'s Android operating system, which has emerged as a significant competitor to the iPhone. It is also making the Nexus One phone, which Google is selling directly to consumers.
Apple said HTC has infringed on 20 of its patents covering aspects of the iPhone's user interface and hardware.
"We can sit by and watch competitors steal our patented inventions, or we can do something about it," Apple CEO Steve Jobs said in a statement. "We've decided to do something about it."
When the iPhone first came out in 2007, it changed the smart phone landscape by introducing a stylish, easy-to-use device. Apple later followed with an application store that extended the capabilities of the device far beyond just making phone calls, checking e-mails and surfing the Web.
Since the iPhone's debut, Apple has had a lock on much of the smart phone market, alongside Research In Motion Ltd., which makes the popular BlackBerry devices.
However, over the last year or so, more competition has emerged from such phone makers as HTC and Motorola Inc., which are rolling out smart phones that use Google's Android software. Not only do these phones appeal to consumers, but they also work on numerous wireless networks, unlike the iPhone, which is still limited in the U.S. to AT&T Inc.
In a court filing, Apple said that a bevy of HTC-made cell phones - including the Nexus One and T-Mobile's G-1 and the myTouch 3G - are using patent-protected technologies owned by Apple without having a license for them.
Not all of the phones listed in the lawsuit run the Android operating software.
Apple is seeking unspecified damages and court orders to block U.S. sales of HTC's Android phones and other products that Apple says violate its patents.
Sales of HTC products aren't immediately threatened, though. Patent cases can take months or years to resolve, and agreements over licensing and royalty payments often emerge.
The complaints were filed with the U.S. International Trade Commission, which has the power to block imports of products and parts made with contested technology, and the U.S. District Court in Wilmington, Del., which can award damages and order HTC to stop sales.
In an e-mail message, HTC spokeswoman Linda Mills said the company only learned of the lawsuits on Tuesday through media reports and hasn't had time to review Apple's claims.
"HTC values patent rights and their enforcement but is also committed to defending its own technology innovations," Mills said.
Technology companies routinely file complaints against competitors over intellectual property. Apple itself faces litigation over the iPhone and other products from the Finnish cell phone maker Nokia Corp., which claims that Apple is using patented technology that helps cut manufacturing costs, shrink the size of consumer gadgets and preserve battery life. Apple responded to Nokia's complaint by filing a countersuit.
Canaccord Adams analyst Peter Misek noted that Apple has not sued that many companies in the past, so the HTC suit must either be a strategic move or the result of what Apple considers a clear infringement on its patents.
Shares of Apple, which is based in Cupertino, Calif., rose 72 cents to $209.71 in midday trading Tuesday.