Edward Snowden held a secretive invite-only talk at SXSW

Edward Snowden appeared via a video call at the SXSW festival in Austin, Texas on Sunday morning - but the Verge reports that the session wasn't open to the public.

The exiled NSA whistleblower can't attend tech conferences in person because he'll likely be arrested if he leaves Russia. That means that he has to make virtual appearances instead.

The session on Sunday morning saw Snowden speak to a group of invited privacy campaigners and technology experts.

The Verge says that "roughly two dozen people" attended Snowden's SXSW call. It was described as a "call to arms" that came in the form of a question and answer session.

Snowden used the session to call on companies to introduce technology that blocks mass surveillance, and pushed for more investment in web security technology SSL.

SXSW told the Verge that the Snowden call was kept private to create an "intimate" feeling for the call. The attendees included CloudFlare CEO Matthew Prince, Twitter's Matthew Zimmerman, and Evernote CEO Phil Libin.

Snowden also appeared at a conference in London on Saturday - but that didn't go smoothly. He used Google Hangouts for the video call, and the call was left public. That meant that anyone on the internet could join the call and interrupt him, and several people did.

After one intruder loudly swore during the call, Snowden looked annoyed. Eventually he got through the panel discussion without too much disruption.