Police have fired teargas and water cannons at protesters who were shouting "Down, Down, Hosni Mubarak" and stamped on posters of the president in demonstrations after Friday prayers, witnesses say.
Hundreds of anti-Mubarak protesters have taken to the streets in central Cairo and other cities including Suez, throwing stones and dirt at the police.
Security forces have mounted a clampdown in Cairo and blocked internet sites prior to the demonstrations.
Ahead the latest protests, trucks of police had lined side roads leading to Tahrir, a square in Cairo where some of the biggest demonstrations and most violent clashes had been held earlier this week.
At the Mostafa Mahmoud mosque in the upscale Cairo suburb of Mohandiseen, police and security officers have lined up preventing cars from crossing towards the mosque.
Internet via Egyptian servers was blocked across the country shortly after midnight (local time), closing a key tool for activists relying on social media networks.
Mobile phone and text messaging services also appear to be disabled or working sporadically.
Facebook has been the main vehicle for announcing the latest protest and identifying locations for demonstrations.
Emboldened by this month's revolt that toppled the leader of Tunisia, Egyptians have staged mass protests since Tuesday (local time) and hundreds have been arrested.
UN Secretary-General Ban Ki-moon has appealed for Egypt's leaders and its people not to let violence escalate.
"All concerned, people or leaders, should ensure that the situation in that region, particularly in Egypt should not lead to further violence," he said.
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(Last night) Obama calls on Egypt to avoid violence:
