Amnesty International has warned that the internet "could change beyond all recognition" unless action is taken against the erosion of online freedoms.
Burma is among countries accused of repressing net freedoms
The warning comes ahead of a conference organised by Amnesty, where victims of repression will outline their plights.
The "virus of internet repression" has spread from a handful of countries to dozens of governments, said the group.
Amnesty accused companies such as Google, Microsoft and Yahoo of being complicit in the problem.
When challenged on their presence in countries such as China in the past, the companies accused have always maintained that they were simply abiding by local laws.
Amnesty is concerned that censorship is on the increase.
"The Chinese model of an internet that allows economic growth but not free speech or privacy is growing in popularity, from a handful of countries five years ago to dozens of governments today who block sites and arrest bloggers," said Tim Hancock, Amnesty's campaign director.
"Unless we act on this issue, the internet could change beyond all recognition in the years to come.
More and more governments are realising the utility of controlling what people see online and major internet companies, in an attempt to expand their markets, are colluding in these attempts," he said.
Amnesty has criticised Google's presence in China
According to the latest Open Net Initiative report on internet filtering, at least 25 countries now apply state-mandated net filtering including Azerbaijan, Bahrain, Burma, Ethiopia, India, Iran, Morocco and Saudi Arabia.
Filtering was only one aspect of internet repression, the group said. It added that increasingly it was seeing "politically motivated" closures of websites and net cafes, as well as threats and imprisonments.
Twenty-two-year-old Egyptian blogger Abdul Kareem Nabeel Suleiman was imprisoned for four years in February for insulting Islam and defaming the President of Egypt.
Fellow Egyptian blogger Amr Gharbeia told the BBC that the internet was allowing people to express themselves: "The web is creating a more open society, it is allowing more people to speak out. It's only natural that upsets some people."
The Amnesty conference - Some People Think the Internet is a Bad Thing: The Struggle for Freedom of Expression in Cyberspace - will have some well-known speakers including Wikipedia founder Jimmy Wales.
It marks the first anniversary of Amnesty's website irrepressible.info, which is being relaunched to become an information hub for anyone interested in the future of internet freedom.