Egyptian security forces are on high alert, with thousands of people expected to join anti-government rallies after Friday prayers.
According to Middle East editor Jeremy Bowen, "today is the test" of the determination of the political opposition.
The protests have led many international leaders to call on President Mubarak to loosen his hold on power.
But Middle East envoy Tony Blair, while he is "absolutely in favour of change" in the country, says the process has to be conducted "very, very carefully".
"If you open up a vacuum anything can happen," he told Justin Webb. "The question is how they evolve and modernise".
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