Some extremist websites urged Muslims to rally after Friday prayers and to back the fundamentalist Muslim Brotherhood, which is the largest opposition group in Egypt.
"On Friday, huge crowds should emerge from mosques and the Islamists should unite and work together with other parties and leaders of the protests," said a statement posted on Muslim.Net, a website associated with al-Qaida.
The Muslim Brotherhood, which is officially banned and calls for rule by Islamic law in Egypt, has joined the protesters in calling for Mubarak's ouster.
The group has been reluctant to present itself overtly as a driving force in the protests. But some fear it could threaten U.S. interests on issues ranging from Arab-Israeli peace efforts to counterterrorism if it gains power.
While the Brotherhood claims to have closed its paramilitary wing long ago, it has fought politically to gain power. It has also built a nationwide charity and social network that much of Egypt's population depends on for survival because of widespread poverty and problems with basic services - complaints at the center of the uprising.
"We call upon the Islamists to support the Muslim Brotherhood because it is the most organized group to lead the Islamic movement and to take power," the statement posted by a commentator on the Muslim.net website said.
Evan Kohlmann, a terrorism analyst with Flashpoint Global Partners, a New York-based security consultancy, said online extremists have rallied behind the protests.
But he said many al-Qaida loyalists were actually skeptical about the role of the Muslim Brotherhood, which has cast itself in an uneasy partnership with pro-democracy advocate Mohamed ElBaradei and other opposition groups in Egypt.
"I'm also seeing quite a bit of scorn for the so-called 'leading role' being played by the Muslim Brotherhood," Kohlmann said Thursday in an e-mail. "I don't think it has helped matters that the Brotherhood have already aligned themselves under a proposed secular unity government" run by ElBaradei.
Protesters in Egypt have been emboldened by a popular uprising in Tunisia that led to the ouster of that country's authoritarian ruler. The militants' statement warned that the failure of Arab governments to implement economic and political reforms will lead to more revolts in countries such as Syria, Algeria, Yemen and Libya.
And there are already signs of unrest. Tens of thousands of protesters rallied in Yemen's capital Thursday calling for the ouster of President Ali Abdullah Saleh, and Facebook and Twitter campaigns in Syria are calling for large anti-government protests in Damascus on Friday and Saturday.
One Egyptian militant leader, Tharwat Salah Shehata, urged the protesters in Cairo not to be fooled by government moves such as Cabinet changes or price reductions. He also expressed regret that he and his fellow Egyptian fighters couldn't directly participate because they were in prison or forced exile.
"We support our Egyptian people in their heroic stance against tyranny and we call upon them to continue their efforts to topple the pharaoh and his aides and to clean our Egypt from their rule that brought hard times, torture, hunger and humiliation to this nation," he said in a statement posted on a militant website.
The subject also dominated other discussion forums.
One writer called upon the mujahedeen, or holy warriors, to "reap the fruits" of revolutions that will weaken the control of the government and security forces, according to a translation by the SITE Intelligence Group, which monitors extremist websites.
"Jihadists may then leap on that throne if they work on the ground and create the factors for this, and a benefit of this is the loss of the striking iron fist from the security services of the government and others that is oppressing the jihadi movement," the posting said.
Kohlmann said the real concern was likely to be a vacuum of power left by a violent transition of power that could "open the door to al-Qaida and other militant groups, in the same way it did in the freewheeling post-Saddam era in Iraq."
Kim Gamel contributed to this report from Baghdad.