Nine people have been reported hurt in violence in Cairo as supporters and opponents of President Muhammad Morsi prepare for rival rallies.
Egyptian media said assailants fired shots and threw Molotov cocktails at protesters in Cairo's Tahrir Square early on December 11.
Opposition groups have called for protest marches in Cairo to force the president to cancel a referendum on a controversial new constitution planned for December 15.
Morsi's Islamist allies have called for a demonstration in support of the vote.
The rival rallies come one day after Morsi ordered the military to maintain public order jointly with the police. He also granted military forces the authority to arrest civilians.
The draft constitution, written by an assembly dominated by the president's Islamist allies, has been condemned by opposition and human rights groups for failing to guarantee fundamental freedoms and protections for women and minorities.
Morsi and his supporters say the new constitution is needed to secure the changes in Egypt since last year's ouster of former President Hosni Mubarak.
Protesters on December 11 sounded determined to force the cancelation of the upcoming referendum.
"We will try not to reach Saturday without achieving anything," one protester said. "We will die in order to make Saturday not come to Egypt because if Saturday comes and we are still the same, then God be with Egypt."
It is still unclear whether Egypt's main opposition group, the National Salvation Front, will call for a boycott or a "no" vote if the December 15 referendum went ahead as planned.
In an interview with the BBC, one of the front's leaders, Muhammad ElBaradei, said he wanted the referendum postponed.
"This is a constitution that defies our basic rights and freedoms. It doesn't establish a proper democratic system," the former head of the UN's nuclear watchdog and Nobel Peace Prize laureate said.
"So, we are, at this stage at least, deciding that we will continue to fight with tooth and nail against the referendum next Saturday."
The constitutional referendum has sparked the most serious political crisis in Egypt since Mubarak's ouster in February 2011.
At least seven people were killed and hundreds were injured in clashes between supporters and opponents of the president last week.
With reporting by Reuters, BBC, and dpa