Crowds have been gathering in Cairo’s Tahrir Square ahead of a mass rally to demand the resignation of Egypt’s Islamist President Mohammed Morsi.
Thousands spent the night in the square, focus of the protests which brought down ex-leader Hosni Mubarak.
Protests are also expected at the presidential palace in Cairo and across Egypt on the first anniversary of Mr Morsi’s inauguration as president.
His opponents accuse him of failing to tackle economic and security problems.
Supporters of the president also plan to hold their own rallies.
Egyptians have been talking about 30 June for many weeks – the opposition vowing not to leave until Mr Morsi steps downed and calls early presidential elections, says the BBC’s Aleem Maqbool in Cairo.
But there is a big part of this society that says that Mr Morsi is someone who was elected and should see out his full term in office, so there is a real split in this society at the moment, a split that we will see reflected on the streets, our correspondent adds.
Protesters are unhappy with the policies of the Islamist president and his Muslim Brotherhood allies.
Opposition activists say more than 22 million people have signed a petition seeking a snap election. They have urged the signatories to come out on Tahrir Square.
The grassroots movement Tamarod (Rebellion) is behind the petition, which has united liberal and secular opposition groups, including the National Salvation Front.
However, many ordinary Egyptians – angered by Mr Morsi’s political and economic policies – are also taking part in the rally.
Flags and tents form a base camp on the square from where protesters plan to march the presidential palace.
Hanan Bakr travelled specially from Dubai where she lives to join Sunday’s protest.
“I’m hoping to stay on the streets until the whole regime of the Brotherhood is brought down,” she told the BBC.
“We are seeking the support of the country to stand behind the second Egyptian revolution. If Egypt falls under Islamist extremism, this will affect the whole region… Egypt is for all religions – I am a Muslim who attended an Armenian Catholic school.”
Speaking in South Africa, US President Barack Obama urged “all parties to make sure they are not engaging in violence and that police and military are showing appropriate restraint”.
At least three people, including a US citizen, died in unrest on Friday.
Washington has warned Americans not to travel to Egypt.
The UK urged its citizens to “avoid all demonstrations and large gatherings” while France said citizens should “limit movements to those strictly necessary”.
“We would like to see the opposition and President Morsi engage in a more constructive conversation about [how] to move their country forward,” he said.
On Friday, US national Andrew Pochter and another man were killed in the northern Egyptian city of Alexandria as protesters stormed an office of the Muslim Brotherhood.
Mr Pochter, who was in the country to teach English to children and improve his own Arabic, was killed apparently while using a mobile phone to take pictures.
His family said in a statement that he had been stabbed by a protester while observing demonstrations.
The other fatality in Alexandria on Friday was an Egyptian man who was shot dead, according to medical sources.
Another man, said to be a journalist, was killed by an explosion in Port Said and five other people were injured.
President Morsi earlier this week offered a dialogue – a move rejected by his opponents.
Mr Morsi, who hails from the Muslim Brotherhood, became Egypt’s first Islamist president on 30 June 2012, after winning an election considered free and fair.
His first year as president has been marred by constant political unrest and a sinking economy.