According to its statement on the Pastebin website, the group is acting against what it claims are various acts of censorship by the Malaysian Government, and also because of the recent move to block filesharing sites, like the Pirate Bay, in Malaysia.
Anonymous also accused the Government of censoring movies and television shows, and for restricting the Internet, which it says is a basic human right.
“We fear that if you make further decisions to take away human freedom, we are obligated to act fast and have no mercy,” the group said in its statement on Tuesday.
The message was also uploaded on video-sharing site YouTube.
It featured a digitally-generated voice that read out the group's message.
Anonymous will launch the attack at 7.30pm GMT on Wednesday (3.30am Thursday Malaysian time) and has named it “Operation Malaysia.” It posted the threat in a graphic on this website.
Mikko Hypponen, chief research officer at the F-Secure Corporation - a computer security software company based in Helsinki, Finland - tweeted about the threat at 4.42am Malaysian time Tuesday.
Meanwhile, online news site Epoch Times reported that Anonymous selects target countries based on input from users in those countries.
However, the report said, the hacker group has declined to say if there are any Anonymous members in Malaysia.
In the exclusive interview with Epoch Times, Anonymous said it expects the Malaysian Government to “not be happy” over the coming attack.
According to the report, Anonymous has reminded its members taking part in Operation Malaysia to use Virtual Private Networks to encrypt their network connections, and to transfer data from remote locations that cannot be accessed without authentication.
This would help protect the identities of the hackers and their locations.
The attack on the Malaysian Government portal would be in the wake of several earlier attacks by Anonymous on Turkish Government websites for alleged Internet censorship.
According to recent news reports, the Turkish Government has arrested 32 people, suspected to be members of Anonymous, after “Operation Turkey” disrupted a number of official websites in that country.