Supporters of ousted President Mohammed Mursi of the Muslim Brotherhood are protesting in Cairo, calling for him to be reinstated.
It comes more than a week after the army toppled Egypt's first elected leader on a wave of demonstrations.
Crowds swelled at a Cairo mosque as people were bussed in from the provinces, where the Muslim Brotherhood has strongholds.
Meanwhile, the youth-led Tamarud group, which brought millions of people to the streets to demand Mr Mursi resign, has called for a Ramadan celebration in Tahrir Square.
Officials say Mr Mursi is still being held at the Republican Guard compound in Cairo.
Troops killed 53 Islamist protesters there on Monday in violence that intensified anger his allies already felt at the military's decision to oust him.
Four members of the security forces were also killed in that confrontation, which the military blames on "terrorists".
Mursi supporters call it a massacre and say those who died were praying peacefully when troops opened fire.
Many of Egypt's 84 million people have been shocked by the shootings, graphic images of which have appeared on state and private news channels and social media.
The incident occurred just three days after 35 people were killed in clashes between pro- and anti-Mursi demonstrators across the country.
The Brotherhood contends it is the victim of a military crackdown, evoking memories of its suppression under Hosni Mubarak, whose 30-year rule collapsed in an uprising in 2011.
But many of its opponents blame Islamists for the violence, and some have little sympathy for the demonstrators who died, underlining how deep the fissures in Egyptian society are.
Security fears spread to other areas
Underlining the level of concern overseas at Egypt's crisis, two US Navy ships patrolling in the Middle East moved closer to Egypt's Red Sea coast in recent days, in what appeared to be a precautionary move following Mr Mursi's overthrow on 3 July.
The United States often sends Navy vessels close to countries in turmoil in case it needs to protect or evacuate US citizens or give humanitarian assistance.
The unrest has also raised fear over security in the lawless Sinai peninsula bordering Israel and Gaza.
One Egyptian policeman was killed and another wounded early today when militants fired rocket-propelled grenades at checkpoints in the Sinai town of El Arish.
Egyptian state media said police arrested three Palestinian militants for attempted attacks in Sinai.
Charges expected against Mursi
Judicial sources say authorities are expected to charge Mr Mursi, possibly for corruption or links to violence.
Prosecutors are also taking a fresh look at an old case over a 2011 prison break when Mr Mursi was among Brotherhood figures who escaped after being rounded up during anti-Mubarak protests.
The army said it was enforcing the nation's will after millions of people, fed up at economic stagnation and suspicious of a Brotherhood power grab, took to the streets at the end of June to demand his resignation.
However, the detentions and threats of arrest have drawn concern from the US, which has walked a semantic tightrope to avoid calling Mr Mursi's removal a military coup.
US law bars aid to countries where a democratic government is removed in a coup.
The US, which gives Egypt's military $1.3bn in aid each year, has said it is too early to say whether Mr Mursi's removal by the army meets that description.
State Department spokeswoman Jen Psaki said on Wednesday Mr Mursi's government "wasn't a democratic rule".
Her words were warmly received by the interim government and swiftly denounced by the Brotherhood.
Yesterday, Ms Psaki expressed concern over the crackdown on Brotherhood leaders.
"If politicised arrests and detentions continue, it is hard to see how Egypt will move beyond this crisis," she said.
Elections crucial to stability
Crucial to longer-term stability will be holding parliamentary and presidential elections, which the transitional authorities are hoping to achieve in a matter of months.
Adli Mansour, the interim president named by the general who removed Mr Mursi, has announced a temporary constitution, plans to amend it to satisfy parties' demands and a faster-than-expected schedule for parliamentary elections in about six months.
He has named liberal economist Hazem El-Beblawi as interim prime minister.
Mr Beblawi said he would start contacting candidates for ministerial posts on Sunday and Monday, with a view to swearing in a cabinet next week.
Negotiations are difficult, with the authorities trying to attract support from groups that range from secularists to ultra-orthodox Muslims, nearly all of whom expressed deep dissatisfaction with elements of the interim constitution.
Rich Gulf states have thrown Egypt a $12bn lifeline in financial aid, which should help it stave off economic collapse.
More than two years of turmoil have scared away tourists and investors, shrivelled hard currency reserves and threatened Egypt’s ability to import food and fuel.