New Leak Indicates U.S. and Britain Eavesdropped at "09 World Conferences A new set of classified documents disclosed Sunday suggested that Edward J. Snowden, the former National Security Agency contractor who has provided a trove of documents to The Guardian newspaper, had obtained a wider range of materials about government surveillance than had been known, including one document revealing how American and British intelligence agencies had eavesdropped on world leaders at conferences in London in 2009. The latest disclosures, appearing again in The Guardian, came the night before a meeting of the Group of 8 industrialized nations was to open in Northern Ireland, where some of the leaders who were intelligence targets four years ago will be in attendance. The newspaper reported Sunday night that Government Communications Headquarters, or G.C.H.Q., the British eavesdropping agency that works closely with the N.S.A., monitored the e-mail and phones of other countries" representatives at two London conferences, in part by setting up a monitored Internet cafe for the participants. In addition, the United States intercepted the communications of Dmitri A. Medvedev, then the Russian president and now the prime minister, the newspaper said. The Guardian posted some G.C.H.Q. documents on its Web site with part of the contents blacked out. A spokesman for The Guardian said Sunday that the paper decided to redact the documents, and that enough was published "to show the authenticity of the report." The documents indicated that e-mail interception and key-logging software was installed on the computers in the ersatz Internet cafe, that foreign diplomats" BlackBerry messages and calls were intercepted, and that 45 analysts tracked who was phoning whom at the meeting. Richard J. Aldrich, a professor of international security at the University of Warwick and the author of a history of the G.C.H.Q., said the logos of the N.S.A. and Canadian intelligence on one of the British documents suggested that they were accessible to Mr. Snowden "under the auspices of a joint program." He said Mr. Snowden's leak showed that British and American diplomats and politicians got a real-time feed of intelligence on their counterparts at major summit meetings. "Now this is integrated into summit diplomacy, almost like a newsreader getting a feed in their ear," he said. American intelligence officials have expressed alarm at the variety of highly classified material Mr. Snowden obtained, suggesting that his actions revealed a shocking breach in the fundamental principle that intelligence officers should have access only to the material they need to do their jobs. On Sunday, a spokesman for the British foreign service said he would not comment on intelligence matters. Mr. Snowden, 29, who left the N.S.A. station in Hawaii this spring and is now thought to be hiding in Hong Kong, delivered hundreds of N.S.A. documents to The Guardian and The Washington Post. Their initial reports covered the routine collection of data on all phone calls handled by the major American telephone companies and an N.S.A. program called Prism that collects the e-mails and other Web activity of foreigners using major Internet services like Google, Yahoo and Facebook. Disclosures linked to Mr. Snowden now rank among the most significant breaches in the strict secrecy of the N.S.A., the largest American intelligence agency, since its creation in 1952. It suffered a handful of defections during the cold war; more recently, insiders have revealed warrantless eavesdropping inside the United States. By contrast, the latest disclosures have exposed surveillance approved by the Foreign Intelligence Surveillance Court and shared with Congress. A letter delivered to Congress on Saturday from the office of James R. Clapper Jr., the director of national intelligence, said that the surveillance programs had helped thwart "dozens" of terrorist plots in the United States and more than 20 other countries. While the N.S.A. collects and stores the phone records of millions of Americans each year, it examines the records only when there is suspicion of a connection to terrorism, the letter said, adding that in 2012, fewer than 300 phone records were reviewed. The Guardian's latest reports offered a rare window onto the everyday electronic spying that the agency does in close cooperation with Britain, Canada, Australia and New Zealand. Matthew M. Aid, an intelligence historian in Washington, said the reports have "confirmed longstanding suspicions that N.S.A's surveillance in this country is far more intrusive than we knew." He added, "We desperately need to have a public discussion about the proper limits on N.S.A." But he said the reports of spying on world leaders, while distressing to the eavesdroppers because it will make their targets more wary, contained no surprises. "This is just what intelligence agencies do - spy on friends and enemies alike," he said. Only because the shroud of secrecy that covers all of N.S.A. operations is so thick does a glimpse like this come as a shock. While some members of Congress have raised questions about the sweep of the N.S.A."s collection of data on Americans, leaders of both parties have defended the programs and denounced Mr. Snowden before The Guardian published its latest report. Appearing on "Fox News Sunday," former Vice President Dick Cheney praised the agency and called Mr. Snowden a criminal and a traitor. "I think it's one of the worst occasions in my memory of somebody with access to classified information doing enormous damage to the national security interests of the United States," he said. The White House chief of staff, Denis McDonough, appearing on "Face the Nation" on CBS, said leaking information about American surveillance "in effect gives the playbook to those who would like to get around our techniques and our practices, and obviously that's not in our interest in the long haul." John M. Broder contributed reporting from Washington, and John F. Burns from London.