Apple continues to face pressure to open up its digital rights management (DRM) system in order that iTunes tracks will play on digital music players that use other company's DRM technologies.

The weekend saw a number of protests outside Apple retail stores across the US. Meanwhile in Europe, regulators from Denmark and Sweden joined their counterparts in Norway and France to refer iTunes to their respective consumer ombudsman.

In a sense, Apple is being damned for its own success in creating a digital music ecosystem that consumers are happy to use. All forms of DRM in music sales are there entirely at the behest of the record labels. True interoperability between services and devices would require that DRM systems be abandoned in their entirety.

Despite this, Swedish, Danish and Finnish regulators consider iTunes' terms and conditions to be illegal in their countries. Apple also faces pressure in France. In the UK, representatives of the record labels (who insist that DRM is neccessary) complained to a government body last week, demanding that Apple open up its DRM.

Swedish regulator Consumer Agency spokeswoman Marianne Aabyhammar said: "If iTunes fails to improve its terms and conditions in Sweden, we may take the case to Sweden's market court."

This activity has travelled across the Atlantic into the US, where a consumer group called Defective By Design launched a wave of 'flash mob' protests outside eight Apple retail stores across the weekend.

Protests took place between 10am and noon. Protestors dressed in hazardous-material-protection suits and stood outside the shops carrying placards and handing out leaflets. You can see images and a video of the protestors in action.

The group is perhaps a little more savvy than European regulators. The group contends that by restricting how software or files can be used, DRM-equipped products are "defective by design".

"Our aim is the abolition of DRM as a social practice," the campaign manifesto claims.

The group claim that as the largest purveyor of DRM-protected media, Apple has paved the way for further erosion of user rights.