Many of the demonstrators want President Mohamed Morsi to step down, saying he has failed to tackle Egypt's economic and security problems.
Opponents of the Islamist president were gathering in Cairo's Tahrir Square. Supporters of the president planned their own rallies on the other side of the Egyptian capital, not far from the presidential palace.
Some government backers wore homemade body armour and construction hats and carried shields and clubs —-precautions, they said, against possible violence.
Today marks the one-year anniversary since Morsi took power and the demonstrations reflect the growing polarization of the nation.
The Tamarod, or Rebel, youth movement claims it has a petition with 22 million signatures, which calls on Morsi to step down. Morsi's supporters have questioned the authenticity of the signatures.
Egypt has been roiled by political unrest in the two years since the uprising that ousted autocrat Hosni Mubarak, but the latest round of protests could be one of the largest.
The Egyptian leader and his Islamist allies are in one camp and seculars, liberals, moderate Muslims and Christians are in the other.
In the past week alone, at least seven people have been killed in clashes between the president's supporters and opponents in cities in the Nile Delta, while on Friday protesters ransacked and torched as least five Brotherhood offices across the country.
A 21-year-old American college student was stabbed to death on Friday during a protest outside a Muslim Brotherhood office in Egypt's coastal city of Alexandria. Andrew Pochter had travelled to the country to teach English to children for the summer.
Adding to the tension, eight legislators from the country's interim legislature announced their resignation on Saturday to protest Morsi's policies. The 270-seat chamber was elected early last year by less than 10 per cent of Egypt's eligible voters, and is dominated by Islamists, who support Morsi.