KUALA LUMPUR, Malaysia—Malaysia worked on Wednesday to thwart a planned assault on the government’s official web site by international hacker vigilantes who accuse authorities in this Southeast Asian country of Internet censorship.

A loose-knit activist group that calls itself Anonymous has threatened to target Malaysia’s online network early Thursday in retaliation against a government effort to block access to 10 popular web sites often used to illegally download movies, TV shows and music.

Members of Anonymous have claimed responsibility over the past year for cyberattacks against governments ranging from Spain to Egypt and major companies such as Visa, Mastercard and Paypal.

Malaysian Information Minister Rais Yatim said state-run agencies are “taking the necessary preventive measures” to foil the attack that is being threatened for 3:30 a.m. Thursday Malaysian time (1930 GMT Wednesday).

Rais insisted the government’s move to block the web sites was a legitimate action to curb copyright violations in the entertainment industry. The ban on the 10 web sites has been erratically enforced by Malaysian Internet service providers, with some users still able to access them.

In a statement posted on Youtube and various web sites earlier this week, Anonymous said it wanted to send a message to Malaysia that “acts of censorship are inexcusable.”

“We are obligated to act fast and have no mercy,” it said. “Now we will wash your corruption away so be prepared.”

The group identified its target as www.malaysia.gov.my, a one-stop portal for Malaysian government information and links to various ministries and state-run services. The web site appears to hold no confidential or sensitive data.

The notoriety of Anonymous rose last December when it backed the WikiLeaks organization and rallied supporters to flood the servers of Visa, Mastercard and Paypal with traffic, periodically blocking access to their sites. The companies had severed their links with WikiLeaks after it began publishing a huge trove of secret US diplomatic memos.

Earlier this month, officials in Spain and Turkey detained dozens of suspected hackers with ties to Anonymous. The group had targeted the web site of Turkey’s telecommunications watchdog last week to protest plans to introduce Internet filters. The watchdog said it was prepared and disruption was minimal.