WASHINGTON—Thousands of secret military documents were released Sunday by a Web-based organization, a gigantic leak of classified information that appeared to present a bleak view of the Afghanistan war and could have a profound impact on the public perception of the conflict.

The release of the documents, which were obtained and made public by the website WikiLeaks, evoked the Pentagon Papers, the secret history of the Vietnam War, which when published contradicted the public narrative of that war and played a role in turning public opinion against it.

Coming at a time when President Barack Obama's Afghanistan strategy has come under increasing criticism, the release will likely stoke criticism of the war effort, as well as spark a debate about the manner in which the information was made available.

WikiLeaks allowed three publications, the Guardian newspaper in London, the magazine Der Spiegel in Germany and the New York Times, to have access to the documents for several weeks. Those news outlets released stories in a coordinated manner Sunday.

The documents are mostly raw field reports, some spare, some mundane and others rich with narrative details. Many of the low-level reports are the kind that some intelligence experts consider the equivalent of second-hand rumors, said one U.S. official.