On the horizon are key elections to choose a new lower house of parliament. The opposition is hoping to leverage public anger into a substantial bloc in the legislature, but must still weld together an effective campaign in the face of the Islamists' strength at the ballot box. Last winter, the Brotherhood and Salafis won around 75 percent of the lower house's seats, though the body was later disbanded by court order.
Pending the election of a new lower house, Morsi gave legislative powers to parliament's Islamist-dominated upper house, a normally toothless chamber elected by only about 7 percent of Egypt's 50 million voters in balloting last year.
Friday's protests re-created the tone of the 18-day uprising against Mubarak, including the same chants, this time directed against Morsi: "Erhal! Erhal!" —"Leave! Leave!" — and "The people want to topple the regime."
Clashes erupted outside the presidential palace in Cairo when youths tried to push through a police barricade. In other cities, protesters tried to break into Brotherhood offices as well as government and security buildings.
Clashes between protesters and police outside the state TV building in central Cairo continued into the small hours of Saturday. Some of the protesters held sit-ins in major squares and streets, insisting they would not disperse until Morsi leaves office.
Standing near Tahrir Square, retiree Ahmed Afifi said he joined the protests because he was struggling to feed his five children on less than $200 a month.
"I am retired and took another job just to make ends meet," Afifi said, his eyes filling with tears. "I am close to begging. Under Mubarak, life was hard, but at least we had security. ... The first people hit by high prices are the poor people right here."
Tens of thousands massed in Cairo's Tahrir Square, where the 2011 uprising began, and outside Morsi's palace, where banners proclaimed "No to the corrupt Muslim Brotherhood government" and "Two years since the revolution, where is social justice?" Others demonstrated outside the state TV and radio building overlooking the Nile.
In the Nile Delta towns of Menouf and Shibeen el-Koum, protesters blocked railway lines, disrupting train services to and from Cairo. In Ismailia on the Suez Canal, protesters stormed the building housing the provincial government, looting some of its contents. There were also clashes outside Morsi's home in the Nile Delta province of Sharqiyah.
The demands of the loosely knit opposition were varied. Some on the extremist fringe want Morsi to step down and the constitution rescinded. Others are calling for the document to be amended and early presidential elections held.
"There must be a constitution for all Egyptians, a constitution that every one of us sees himself in," Nobel Peace Prize laureate Mohamed ElBaradei said in a televised message posted on his party's website.
Democracy campaigner and best-selling novelist Alaa al-Aswany marched with ElBaradei to Tahrir. "It is impossible to impose a constitution on Egyptians ... and the revolution today will bring this constitution down," he said.
Morsi's opponents complain that he has kept government appointments almost entirely within the Brotherhood, installing its members to everything from governorships and chiefs of state TV and newspapers, down to preachers in state-run mosques.
Many were also angered by the constitution and the way Islamists pushed it through in an all-night session and then brought it to a swift referendum in which only a third of voters participated. The result is a document that could bring a much stricter implementation of Shariah, or Islamic law, than modern Egypt has ever seen.
Looming over the struggle between the Islamists and opposition is an economy in tatters since Mubarak's ouster. The vital tourism sector has slumped, investment has shriveled, foreign currency reserves have tumbled, prices are on the rise and the local currency has been sliding.