(Sharecast News) - Shares in iPhone maker Apple were firmly lower on Thursday after the Department of Justice sued the tech giant for allegedly violating antitrust laws and preventing smartphone customers from switching to its rivals.

The DOJ claims that Apple has misused its stronghold in the market to block access to iPhone features from developers and other rivals in what it deems as anticompetitive behaviour.

The lawsuit, which Apple says it would "vigorously defend", alleges that the company engaged in a "broad, sustained, and illegal course of conduct" to keep control of the market and limit competition.

Apple has taken measures like blocking software and apps or restricting the functionality of other tech when used in conjunction with the iPhone, like smartwatches made my other manufacturers, the claims detail.

"We allege that Apple has maintained monopoly power in the smartphone market, not simply by staying ahead of the competition on the merits, but by violating federal antitrust law," according to a statement from attorney general Merrick Garland.

"If left unchallenged, Apple will only continue to strengthen its smartphone monopoly."

The stock was down 3.6% at $172.16 by 1106 ET.