Supporters of Egypt's deposed president Mohamed Morsi have called for a "million-person march" against his ouster after authorities warned of "decisive" action if protesters are considered a threat.
Organisers of protests against the military's overthrow of Morsi urged demonstrators to march on security buildings on Monday night and called a march for Tuesday.
In a statement, the Anti-Coup Alliance of Islamist groups urged Egyptians "to go out into the streets and squares, to regain their freedom and dignity - that are being usurped by the bloody coup - and for the rights of the martyrs assassinated by its bullets."
The protest calls, which raises the possibility of fresh confrontations, comes after at least 72 people were killed at a sit-in in support of Morsi on Saturday morning.
The country has been bitterly divided between camps supporting and opposing the July 3 toppling of Morsi and hundreds of thousands have come our for rival mass rallies. Morsi's opponents had accused him of abusing his power and giving too much influence to his Muslim Brotherhood group.
'Honourable sons of the nation'
The Anti-Coup Alliance also called for protesters to march to security buildings in provinces across Egypt on Monday night "to condemn the criminal acts and the firing of live ammunition by the interior ministry at peaceful demonstrators".
Morsi supporters insist he will be reinstated, but the interim government has said it will move to disperse the protests and warned it would take "decisive" action if demonstrators overstepped their bounds.
The military issued its own warning to Cairo protesters in flyers dropped from helicopters early on Monday.
"We call on you not to approach military facilities or units, help us to protect your safety," it said, addressing protesters as "honourable sons of the nation".
A group of Egyptian NGOs issued a statement on Monday calling for interior minister Mohamed Ibrahim to be dismissed over what it called a "massacre," but also urged the Brotherhood to denounce violence.
"The interior minister should be dismissed and held accountable for his actions," the groups wrote, urging Morsi supporters to "take action to persuade their colleagues and leaders to renounce" violence.
In its first comments on the bloodshed, the interim presidency said on Sunday that it was "saddened" by the deaths, but dubbed the protest area where they occurred a "terror-originating spot".
A crackdown on Morsi supporters continued Monday, with the arrest of two leaders of the moderate Islamist Wasat party, president Abul Ala Mady and vice president Essam Sultan.
State news agency MENA said they were being investigated on suspicion of inciting violence and murder.
With tensions rising, EU foreign policy chief Catherine Ashton was due to hold discussions with a range of government and opposition figures.
She was expected to meet interim president Adly Mansour and vice president for international affairs Mohamed ElBaradei, as well as members of the Muslim Brotherhood and the Tamarud group that organised protests calling for his removal.
Ashton's visit comes as Egyptian police detained two leaders of a Muslim Brotherhood-allied party in the latest in a wave of arrests.
Security officials said that Abul-Ela Madi and Essam Soltan, who faced arrest warrants linked to allegations of inciting violence, were found hiding in a home in a Cairo neighbourhood located near the main protest site of Morsi's supporters.
The party condemned the arrest of its leaders, saying such measures exacerbate the crisis and add new obstacles to efforts to build bridges.
In a statement, Ashton said she would be calling for "a fully inclusive transition... including the Muslim Brotherhood".
She added that this process must lead as fast as possible to a constitutional system and free and fair elections and a civilian government
"I will also repeat my call to end all violence. I deeply deplore the loss of life," she said.
The Brotherhood's political arm, the Freedom and Justice Party, confirmed it would meet with Ashton on Monday on the basis of "constitutional legitimacy and in pursuit of an end to the military coup".
The group has insisted that it will not accept any solution to the crisis that does not involve Morsi's return to office.