The first batch released Monday suggests that Dow Chemical, the parent company of the firm responsible for the 1984 Bhopal gas tragedy, paid Stratfor to monitor campaigners for victims of the disaster, WikiLeaks claims.
The whistleblowing website said emails also indicate that Coca-Cola paid Stratfor to investigate animal rights group Peta.
A "substantial" proportion of Stratfor's funding comes from government agencies including the US Department of Homeland Security, WikiLeaks said, while adding that it could not yet give exact figures.
WikiLeaks is sifting through the emails, dating from July 2004 to December 2011, with 25 media partners including Rolling Stone magazine and Italy's La Repubblica newspaper, while members of the public can also view the messages.
The emails document "the private lives and private lies of private spies," WikiLeaks founder Julian Assange said at a press conference in London.
"Over the last ten years the private intelligence industry has boomed in the United States and other countries.
"But with this growth there has not been a commensurate growth in accountability mechanisms," added Assange, who is currently fighting extradition from Britain to Sweden over sexual assault allegations.
Stratfor, founded in 1996, slammed the publication of the emails but said it would not comment on their content.
"Having had our property stolen, we will not be victimised twice by submitting to questioning about them," the company said in a statement.
"The emails are private property. Like all private emails, they were written casually, with no expectation anyone other than the sender and recipient would ever see them. They should be read as such," it said.
"Stratfor will not be silenced and will continue to publish the geopolitical analysis our friends and subscribers have come to rely upon."
Assange said the emails contained evidence that Stratfor analysts had monitored WikiLeaks itself, and he promised that details would be revealed in the next few days.
The emails are widely believed to have been passed to WikiLeaks by hacker group Anonymous, which claimed in December that it had stolen them.
But Assange refused to specify how his website had come to possess the messages, saying: "As a matter of policy we don't discuss sourcing or speculate on sources."
"WikiLeaks, in its sourcing methodology, deliberately tries to not even know itself where its information comes from, because ultimately that is the strongest protection."
The white-haired former hacker criticised Statfor's own protection of informants, describing the firm's analysts as "people playing James Bond in the most absurd manner, and in many cases the most ineffective, hopeless manner."
WikiLeaks itself came under fire last September for potentially endangering the lives of government informants by publishing an unredacted version of its archive of 251,000 secret US diplomatic cables.
Assange is awaiting a judgment from the Supreme Court in London on whether he can be extradited to Sweden for questioning over allegations of rape and sexual assault, which he denies.
WikiLeaks has long expressed concern that if Assange is extradited to Sweden, he could eventually be sent on to the United States where he could face prosecution for publishing the diplomatic cables.