Apple has been accused by lawyers in a court case of deleting songs from rival services from some iPods during the past decade.
Users with non-iTunes music received a message telling them to restore devices to their factory settings when they tried to sync them, the court heard. Apple said that the move was a legitimate security measure.
The competition case is examining whether Apple tried to lock down its iPod and iTunes market in 2007-09. The class action lawsuit, brought by individuals and businesses, is being heard in a US district court in California, the BBC reports.
They accuse Apple of abusing a monopoly position in the digital music player market. The case has been going on for more than a decade and could see Apple pay out $US1 billion in damages
Apple's security director Augustin Farrugia said the company's attempt to keep iPods clear of any non-iTunes music was done to protect consumers from hackers and malicious content.
He added that the error message that appeared when users tried to sync the content of an iPod to an iTunes account was vague because the firm did not want to "confuse users" with too much information.
Earlier the court saw the contents of an email that then Apple chief executive Steve Jobs sent in 2005 after learning that a rival company was about to introduce a programme that would let music fans buys songs anywhere and play them on iPods.
"We may need to change things here," the email read.
Lawyers argue there was an internal campaign to keep Apple's iPods free of music that was not purchased from the iTunes store. By updating the iTunes and iPod software to block music from competing online stores, Apple operated a closed system which froze rivals out of the market, they say.
Later in the trial, jurors will see video testimony from Steve Jobs, filmed six months before he died.