In a statement posted on the WikiLeaks site, Assange said the court martial which began near Washington yesterday did not represent justice for the 25-year-old army private.
"This is not justice; never could this be justice. The verdict was ordained long ago," he wrote.
"Its function is not to determine questions such as guilt or innocence, or truth or falsehood.
"It is a public relations exercise, designed to provide the government with an alibi for posterity. It is a show of wasteful vengeance; a theatrical warning to people of conscience."
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He claims the allegations are politically motivated and linked to the release by WikiLeaks of the classified documents allegedly leaked by Manning, which caused huge embarrassment to the US government.
Assange condemned the "abuse" of Manning, who he described as "the most prominent political prisoner in modern US history".
The soldier's lawyers have successfully argued that he has been subjected to unduly harsh detention methods since his arrest while serving in Iraq in 2010.
The Australian former computer hacker said the pre-trial hearings had "comprehensively eliminated any meaningful uncertainty, inflicting pre-emptive bans on every defence argument that had any chance of success".
"It is fair to call what is happening to Bradley Manning a 'show trial'."
Manning has pleaded guilty to several offences but denies the most serious charge -- that he knowingly aided the enemy, chiefly al-Qaeda and late terror chief Osama bin Laden.
Assange said the documents leak to Wikileaks was "the single most important disclosure of subjugated history, ever".