Several hundred demonstrators, mostly young men, clashed with police in the streets off Tahrir Square in central Cairo as night fell on Saturday.
The square, which was the focal point of the uprising in early 2011 that overthrew the Mubarak regime, has seen the ebb and flow of demonstrators over the last five days.
Initially they were out to mark the anniversary of week-long violence that saw some 42 die and to demand the security forces be held accountable.
However, fresh anger erupted following a decree made by President Mohamed Morsi on Thursday extending his powers and shielding him from judicial challenge.
Police used tear gas to disperse crowds in Tahrir Square, where tens of thousands gathered the day before with some of the demonstrators remaining overnight. Al Arabiya television said Morsi supporters attempted to storm the headquarters of the judiciary in Cairo and clashed with opposition protesters there.Bloomberg contributed to this report