The release began at midnight with WikiLeaks warning that details relating to numerous security companies, including defence contractor Lockheed Martin, and US defence agencies would be published.
Founder Julian Assange said: “Today Wikileaks started releasing over 5 million emails from private intelligence firm Stratfor based in Texas, the United States.
“Together with 25 other media partners from around the world we have been investigating the activities of this company for some months. “And what we have discovered is a company that is a private intelligence Enron.
“On the surface it presents as if it’s a media organisation providing a private subscription intelligence newsletter.
“But underneath it is running paid informants networks, laundering those payments through the Bahamas, and through Switzerland, through private credit cards.
“It is monitoring Bhopal activists for Dow Chemicals, Peta activities for Coca-Cola.
“It is engaged in a seedy business.”
Mr Assange said Stratfor was using the secret intelligence it had paid for to invest in a wide range of “geopolitical financial instruments”.
“This makes News of the World look like kindergarten,” he added.
So far, around 200 emails are believed to have been released.
The messages are said to have been obtained through a hacking attack on Stratfor by hacking group, Anonymous.
Stratfor said the release of the emails was an attempt to silence and intimidate it.
Some of the emails being published “may be forged or altered to include inaccuracies. Some may be authentic,” the company said in a statement.