Crowds are gathering in Cairo's Tahrir Square to demand the resignation of Egypt's Islamist President Mohammed Morsi, a year after he came to power.
Thousands of people spent the night milling in the square which was the focus of protests which brought down his predecessor, Hosni Mubarak.
Sunday is the first anniversary of Mr Morsi's inauguration as president.
Tension has been high ahead of the rally. At least three people, including a US citizen, died in unrest on Friday.
The BBC reports 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".
Protesters are unhappy with the policies of the Islamist president and his Muslim Brotherhood allies.
Thousands of supporters of Mr Morsi, who was elected by a small margin, rallied in the capital on Saturday.
Obama calls for dialogue with Egypt
US President Barack Obama has called on Egypt's government and opposition to engage each other in constructive dialogue and prevent violence spilling out across the region.
Bloodshed on Friday killed at least three people, including an American student. Tens of thousands from both sides rallied again on Saturday across Egypt, although there were fewer reports of violence.
Hundreds have been wounded and at least eight killed in street fighting for over a week as political deadlock deepens.
Washington is pulling non-essential staff out of Egypt and Mr Obama says he is looking at the situation with concern.
"Every party has to denounce violence," Mr Obama said during a visit to South Africa. "We'd like to see the opposition and President Morsi engage in a more constructive conversation about how they move their country forward because nobody is benefiting from the current stalemate."
He added that it was "challenging, given there is not a tradition of democracy in Egypt".
Mr Morsi's critics have dismissed US calls for restraint as a sign of Washington backing the Egyptian president, just as it backed Hosni Mubarak before he was deposed by people power in early 2011, Reuters reports.
They accuse his Muslim Brotherhood of hijacking the revolution and using electoral majorities to monopolise power, and hope millions will march to demand new elections on Sunday.