Caspar Bowden says he was unaware of Prism data-sharing program when he worked at software firm

Microsoft's former chief privacy adviser said he did not have faith in the security of the software company's technology during a lecture on revelations about the US's NSA spy agency published in the Guardian.

"Caspar Bowden, who between 2002 and 2011 gave advice on privacy to Microsoft's national technology officers in 40 countries in which Microsoft operated – but not the US – told a conference in Lausanne, Switzerland, that he was unaware of the Prism data-sharing program when he worked at the company.

"I don't trust Microsoft now," he said, adding that he only uses open source software where the underlying code can be examined. He also said he has not carried a mobile phone for two years.

In June the Guardian revealed that an NSA program called Prism could demand data from a number of technology companies at will using court orders that were never rejected.

Bowden said the extent of the NSA's surveillance efforts – where it shares and gathers intelligence with the UK's GCHQ and intelligence agencies in Canada, New Zealand and Australia – was undermining democracy.

"The public now has to think about the fact that anybody in public life, or person in a position of influence in government, business or bureaucracy, now is thinking about what the NSA knows about them. So how can we trust that the decisions that they make are objective and that they aren't changing the decisions that they make to protect their career? That strikes at any system of representative government."

The wording of the Foreign Intelligence Surveillance Act (Fisa) under which the NSA gathers intelligence means that "there's no protection if you're not an American", said Bowden.

He added: "We're living through a transformation in surveillance power that's never been seen before on earth. And we don't know what type of government or leader will come to power next and exploit it. It could be the next president. It could be this one."

Another speaker at the conference, digital activist Jacob Appelbaum, who has worked with WikiLeaks and on the Tor anonymity system, suggested that some employees of the NSA should be arrested if they visit Europe on the basis that by deliberately weakening cryptographic systems they had put people in danger.

Earlier this month the Guardian explained how NSA and GCHQ have worked to insert mathematical weaknesses into cryptography systems used to scramble internet data and other information.

That has put people at risk of their lives, Appelbaum argued. "People who commit mass human-rights violations, they should be prosecuted," he said. "The NSA has a slogan internally — 'we track 'em, you whack 'em' – where they help to target drone strikes." Such strikes were an abuse of natural justice, he said.

• This article, including the headline, was amended on 7 October 2013. An earlier version gave the impression that Bowden's distrust of Microsoft's technology did not begin until after the NSA revelations. In addition we also suggested that he was in charge of the company's privacy policy and that he now trusts open source software where he can examine the underlying code himself. All three errors have been corrected.