Supporters of Egyptian President Mohamed Morsi are set to begin an open-ended demonstration on Friday, two days before planned rallies against him, setting up a showdown in the streets in the coming days.
With the start of Egypt's weekend, people began to gather in Cairo's Tahrir Square, site of the uprising of January 25, 2011, and at venues in other towns.
Morsi's legitimacy is at the heart of protests both for and against him.
His supporters say he derives his authority from the first free presidential election in Egypt's history, and that the challenges he faces - corrupt and inefficient institutions, economic troubles and religious tensions - were inherited.
But his critics see him as a Muslim Brotherhood delegate, favouring Islamists in key positions and returning the country to authoritarianism.
The Islamists accuse the opposition of being remnants of ousted president Hosni Mubarak's regime and of seeking to sow chaos.
That accusation was repeated by Morsi in a televised speech on Wednesday, when he warned that political polarisation threatened to "paralyse" Egypt.
He pledged to look into constitutional reforms and also reached out to the opposition again for talks.
But late on Thursday, the opposition National Salvation Front coalition rejected the appeal for dialogue and called instead for an early presidential election.
Since taking office a year ago, Morsi has squared off against the judiciary, media, police and even artists.
In his speech on Wednesday, Morsi threatened legal action against several named prominent figures. He said some judges and civil servants were obstructing him, and accused liberal media owners of bias.
Hours after he publicly accused one TV channel owner of tax evasion, the businessman, Mohamed al-Amin found he was under investigation and barred from leaving the country. Amin's channel notably airs satire modelled on that of US comic Jon Stewart.
Separately, officials ordered the arrest of a talk show host on another channel known for his anti-Islamist diatribes and ordered that station to be shut down for inciting mutiny in the army and for insulting the armed forces and the police.
An anchor on state television resigned dramatically live on air in protest at what he said were attempts by the information minister, an Islamist, to control his programme.
Egypt's powerful army, which has been on the sidelines since Morsi's election, warned it would intervene if violence breaks out.
It has brought in reinforcements to key cities in order to protect vital establishments in case unrest erupts, security officials said.
In Cairo, residents are withdrawing cash and stocking up on food, and many companies have said they will close on Sunday, the first day of the working week in Egypt.
Fuel shortages have seen cars queueing overnight outside petrol stations, bringing parts of the city to a standstill and adding to the tension.