Google chief Larry Page on Friday branded Internet spying a threat to freedoms and called for governments to be more revealing about what they try to find out about people's online activities.

"We understand that the US and other governments need to take action to protect their citizens' safety -- including sometimes by using surveillance," Page said in a blog post.

"But the level of secrecy around the current legal procedures undermines the freedoms we all cherish."

Page put his personal stamp on the California-based Internet giant's denial that it opened any doors for US intelligence agencies to mine data from its servers.

Google and other technology firms on Thursday were adamant that they did not knowingly take part in a secret program called PRISM that gave the National Security Agency (NSA) and the FBI back doors into servers at major Internet companies.

"We have not joined any program that would give the US government or any other government direct access to our servers," Page said.

"Indeed, the US government does not have direct access or a 'back door' to the information stored in our data centers," he continued. "We had not heard of a program called PRISM until yesterday."

The program was reportedly set up in 2007 and has grown "exponentially" to the point where it is now the most prolific contributor to President Barack Obama's Daily Brief, the US leader's top-secret daily intelligence briefing.

Some of the biggest firms in Silicon Valley were involved in the program, including Microsoft, Yahoo, Google, Facebook, Apple, PalTalk, AOL, Skype and YouTube, reports said.

However, Internet titans contacted by AFP denied providing intelligence agencies with back doors to networks and held firm that they only cooperated with legal "front door" requests for information.

"This episode confirms what we have long believed -- there needs to be a more transparent approach," Page said.

Google routinely publishes transparency reports listing numbers of requests for user data by governments and how they were handled.