The group said Monday it had broken into computer systems of Shell, BP Global, ExxonMobil, Gazprom and Rosneft and posted hundreds of company e-mail addresses and passwords online.
Information from 190 Gazprom accounts and 80 from Rosneft were posted, as well as additional database access details, The Moscow Times reported.
The cyberattack was carried out as the environmental activist group Greenpeace launched a protest action to shut down Shell gas stations in England and Scotland.
In the posting, Anonymous quoted a Greenpeace petition that seeks an end to the "new oil rush" in the arctic. However, it said its cyberattack on the five oil companies was an independent action.
"This Operation is carried out by Anonymous and isn't anyhow affiliated to GreenPeace! We are just supporting their cause," the group wrote.
Anonymous said Monday's attack was "Phase II" of an operation begun June 26 when e-mail addresses and passwords from the five companies were similarly published and used to sign the Greenpeace petition to ban oil drilling in the arctic and establish it as a protected world park.
Greenpeace's petition in part reads: "We know we're going up against the most powerful countries and companies in the world. But together we have something stronger than any country's military or any company's budget. Our shared concern for the planet we leave our children transcends all the borders that divide us and makes us -- together -- the most powerful force today."
The energy companies didn't comment on the Anonymous cyberattack.
Holding an estimated one-fifth of the world's undiscovered, recoverable oil and natural gas, the arctic has become the focus of plans by the major oil companies, who are looking to replace dwindling supplies elsewhere.
But environmentalists say an oil spill in the arctic would pose monumental challenges and threaten vast harm to the pristine region.
The cyberattack came as Greenpeace activists shut down 74 Shell gas stations in Edinburgh and London in actions that resulted in 24 arrests, police said. Protesters tried to cut off gasoline supplies to London's 105 Shell stations and Edinburgh's 14 stations, The Guardian reported.
As of late Monday, 18 protesters in London and six in Edinburgh had been arrested. Edinburgh police had reportedly parked cars outside all Shell stations across the Scottish capital.
"It's time to draw a line in the ice and tell Shell to stop," Greenpeace activist Sara Ayech told the newspaper. "That's why today we're going to shut down all of Shell's petrol stations in the capital cities of London and Edinburgh. We've got dozens of people who will hit over 100 Shell garages throughout the day."
In recent weeks, Anonymous has been active on a number of fronts. This month the group attacked alleged child pornography Web sites, taking down several sites it deemed to be connected to the trading of illegal files and video.
The group also attempted to attack a Japanese government Web site to protest a new law calling for jail time for illegal downloading but ended up crippling the site of small local government water management agency in a case of mistaken identity.
"We made a mistake. We're sorry. Japanese is difficult," the group says in a Twitter message.