The online group of hactivists known as "Anonymous" expressed their support for protesters in Egypt Wednesday by calling for cyber attacks on websites run by the Egyptian government.

Tens of thousands of demonstrators took to the streets across Egypt this week, facing down a massive police presence to demand the ouster of President Hosni Mubarak in protests inspired by Tunisia's popular uprising.

After reports said that social media websites Twitter and Facebook had been restricted in the country, the "Anonymous" Facebook page "Operation Egypt" issued a dire warning to the Egyptian government.

"To the Egyptian Govt : Anonymous challenges all those who are involved in censorship," the group wrote. "Anonymous wants you to offer free access to uncensored media in your entire country. When you ignore this message, not only will we attack your govt websites, we will also make sure that the international media see the horrid reality you impose on your people!"

A graphic on the Facebook page gave instructions for participating in distributed denial-of-service (DDoS) attacks.

In December, "Anonymous" was successful in taking down Visa, Mastercard and other websites of organizations that refused to do business with secrets outlet WikiLeaks.

While some characterized the DDoS efforts as a form of cyber-terrorism, others noted that many participants consciously opted in to the networks, downloading a piece of software that points at a predetermined server and simply asks it to do what it's made to do: serve pages. When these networks are comprised of volunteers, DDoS attacks are more akin to sit-in protests than terrorism.

More recently, the group got into the business of real-world demonstrations by calling for a global day of protest on Jan. 15 to defend free speech.

Egyptian activists called for a second day of street action on Wednesday as authorities vowed to prevent further protests.

The pro-democracy youth group April 6 Movement, the driving force behind Tuesday's protests -- the largest and most significant in Egypt since bread riots in 1977 -- urged people to head back to Cairo's main square.

Tuesday's demonstrations, dubbed "the day of anger" and inspired by the uprising in Tunisia, left three protesters and one policeman dead, according to medics.

Despite some 20,000 to 30,000 police being deployed in central Cairo, thousands of demonstrators marched to Tahrir Square on Tuesday, where they chanted in unison: "The people want the ouster of the regime."

Video from the tumultuous scene showed protesters actively standing up to mobile water cannons and chasing large squads of riot police through the streets.

-- With earlier reporting by Stephen C. Webster and AFP