Technology has given internet giants "irresistible power" when they work in concert with governments, whistleblower Ed Snowden told the Web Summit that opened in Lisbon on Monday.
"When we see government and corporations working in concert... they become the left and right hands of the same body. What we see is the concentration of power," he told the European celebration of startups and new technologies gathering high-tech entrepreneurs and investors.
"If you create an irresistible power... how do you police the expression of that power when it is used against the public rather than for it?" he asked, speaking by video link from Russia where he has lived since 2013.
The US government last month urged tech giants to allow police to read encrypted messages, saying access was essential to prevent serious crime despite privacy concerns.
Snowden has just published a book that lays out his reasons for passing tens of thousands of secret documents to major news organisations in 2013.
The files were compiled while he worked for the US National Security Agency and revealed a dense network of communications and Internet scrutiny by the NSA and partner agencies around the world.
Snowden recognised that public awareness is growing over the abuses he has denounced, and he lauded efforts to protect privacy, especially in Europe.
But he told the gathering of some 70,000: "The problem is not data protection, it's data collection" and the blind faith that Internet users must have in the Internet's masters.
The four-day summit is expected to focus on politics and tax issues, as well as new mobilities, medical applications, robotics and crypto currencies, organisers said.
"Tech has become hyper-political," said Paddy Cosgrave, the Irish founder and boss of Europe's biggest tech gathering.
"Increasingly, the front page of newspapers around the world are dominated by issues relating to technology," he told AFP.
Among the main events are discussions on the future of money, cars, medicine, housing, advertising, medias and humans' presence in outer space.
But what has emerged as the leading topic is how high tech has become a crucial factor in the Chinese-US trade war, the monetary power of sovereign governments and the radicalisation of social media.
As sector giants continue to face calls for fair taxation or even dismantlement, regulators such as the EU Commission's vice president and competition chief, Margrethe Vestager, are expected to draw a crowd.
Vestager is to close the summit on Thursday, speaking just after Michael Kratsios, who is being sent from the White House to present the US viewpoint on Internet taxation and regulation.
Vestager has spearheaded European efforts to get companies like Amazon, Apple, Facebook, and Google to pay more in taxes in countries where they earn large amounts of their profits.
In addition to her post as EU competition chief, the Dane has also now been tasked with overseeing digital activities across the 28-member bloc.
Vestager "is incredibly popular... because she's trying to create a level playing field for innovators in particular in Europe," Cosgrave told AFP.
At another event, former Cambridge Analytica executive Brittany Kaiser is expected to outline risks to personal data in the run-up to the 2020 US presidential election.
The now defunct data consultancy allegedly hijacked personal data on Facebook users ahead of the 2016 US vote.
Huawei's rotating chairman Guo Ping is another headliner.
He is likely to call for support from the tech community after the Chinese phone giant was banned from the United States owing to suspicion its systems could be used to collect data for Beijing.
A scheduled address almost certain to raise the issue of Internet taxation is by Pascal Saint-Amans, head of the OECD's Centre for Tax Policy and Administration.
The Organisation for Economic Co-operation and Development is drafting a "unified approach" to a digital tax on Internet giants and multinational groups to be presented by June next year.