The presidential palace has been a focus for demonstrations against Mr Morsi
Egypt's capital Cairo is steeling itself for rival rallies sparked by a referendum on a new constitution ordered by President Mohammed Morsi.
The demonstrations have been called by largely secular opposition groups and Islamists backing Mr Morsi.
At least nine people were hurt early on Tuesday when shots were fired at opposition protesters in central Cairo.
President Morsi has called in the army to maintain security and protect state institutions ahead of Saturday's vote.
He has given the army powers to arrest civilians and has tried to calm public anger by annulling a decree boosting his powers.
But some rulings of the 22 November decree - which stripped the judiciary of any right to challenge his decisions - will stand.
The general prosecutor, who was dismissed, will not be reinstated, and the retrial of former regime officials will go ahead. Clashes feared
Petrol bombs were thrown and shots fired at opposition demonstrators camped out in Tahrir square in the early hours of Tuesday. Nine people were wounded in the arms and legs while another protester suffered a head injury, Al-Misri al-Yawm newspaper reported.
After the attack, police cars were deployed around the square for the first time this month.
The focus of Tuesday's opposition protests will be the presidential palace, the scene of earlier demonstrations which has now been surrounded with concrete blocks and ringed with tanks.
Around three miles away, an "alliance of Islamist forces" made of Muslim Brotherhood and Salafist groups are due to gather at two mosques in Nasr City, a suburb of Cairo.
Islamist demonstrators have also been staging a sit-in outside a Cairo media complex that hosts the studios of several private TV channels, which pro-Morsi protesters accuse of bias.
There are fears there could be more clashes during the day if rival groups of protesters come face-to-face, the BBC's Jon Leyne in Cairo reports.
The military presence on the capital's streets has also raised fears Egypt is moving back towards military rule, our correspondent says.
The president says he is trying to safeguard the revolution that overthrew Hosni Mubarak last year, but critics calling for large turnouts at Tuesday's protest accuse him of acting like a dictator.
The opposition National Salvation Front has said it will not recognise the draft constitution, as it was drafted by an assembly dominated by Mr Morsi's Islamist allies.
NSF chief co-ordinator Mohammed ElBaradei said the "sham" draft constitution defied Egyptians' "basic rights of freedom".
"It doesn't establish proper democratic systems, so at this stage at least we have decided that we are going to continue to fight tooth and nail against the referendum," the Nobel prize winner told the BBC.
Mr ElBaradei would not go so far as to call for a boycott of Saturday's vote, but said he hoped the turn-out at Tuesday's protests would persuade Mr Morsi to postpone the referendum until consensus was reached through dialogue on a "proper, democratic" constitution.
Meanwhile, Mohamed Soudan, foreign relations secretary of the Muslim Brotherhood's Freedom and Justice Party, said Mr Morsi was constitutionally bound to go ahead with Saturday's vote because the date had been announced by the constituent assembly.
An umbrella group calling itself the Alliance of Islamist Forces plans to hold counter-demonstrations on Tuesday in support of the referendum and the president, under the slogan: "Yes to legitimacy".
The proximity of the protests has raised fears of further bloody clashes on the streets of the Egyptian capital.
Fighting between rival protesters has killed several people and wounded many more since Mr Morsi signed the decree giving himself sweeping new powers.
The police have been seen as weakened since the fall of former President Hosni Mubarak, correspondents say.
They failed to intervene when anti-Muslim Brotherhood protesters ransacked the Islamist movement's Cairo headquarters last week.