Britain's Guardian newspaper says spies monitored the computers and intercepted the phone calls of foreign politicians and officials at two G20 meetings in London in 2009.
The newspaper says the effort included penetration of delegates' BlackBerry smartphones to monitor their email messages and calls.
The Guardian published PowerPoint slides describing the operation, including one featuring the emblem of the Communications Security Establishment, Canada's electronic eavesdropping agency, known as CSEC.
It appears alongside the logos of key allies — the U.S. National Security Agency and Britain's Government Communications Headquarters — as well as an icon labelled Joint Apps.
The documents were leaked to the Guardian by former NSA contractor Edward Snowden, who has touched off a vigorous global discussion about state surveillance.
Earlier documents from Snowden indicated that a top-secret data-mining program known as Prism has given the U.S. government access to a huge volume of emails, chat logs and other information from Internet companies including Google, Microsoft and Apple.
The latest story appeared as G8 leaders gathered in Britain for a round of talks.
The Guardian says special Internet cafes were set up in 2009, allowing authorities to install an email interception program and key-logging software to spy on delegates' computer use. In addition, a team of analysts had access to summaries of phone calls.
The Turkish finance minister and Russian leader Dmitry Medvedev were singled out for special attention, the paper reported, and it appears information was passed to British cabinet ministers.
"Delivered messages to analysts during the G20 in near real-time," reads the slide with the CSEC logo.
"Provided timely information to UK ministers."
A CSEC spokesman had no immediate comment Tuesday.
The Ottawa-based CSEC has a staff of more than 2,000 — including highly skilled mathematicians, codebreakers and linguists — and an annual budget of about $400 million.
There have been allegations over the years about spying by western intelligence services at global summits, said veteran CSEC watcher Bill Robinson, whose blog luxexumbra.blogspot.ca chronicles the intelligence agency's activities.
"At meetings, it's always been rumoured, or leaked sometimes," he said Tuesday. "It's interesting to see this clear evidence of it.
Robinson said it is very likely that similar surveillance has taken place at other G20 meetings, including the one in Toronto in 2010.
"It's probably being done at the G8 meeting right now," he said.
Robinson said Snowden's revelations have "kickstarted a useful debate."
"Whether it'll continue for much longer I don't know," he said.
"These are important questions that need to be debated — I think about the intrusiveness of these agencies and the balance that has to be drawn somewhere between the value of collecting the information against the value of privacy."
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