CAIRO — At least four people are dead across Egypt, including a 14-year-old boy, after antigovernment protesters ransacked Muslim Brotherhood offices on Friday and tens of thousands of supporters and opponents of President Mohamed Morsi held dueling rallies in the capital, Cairo.

While the protests in Cairo remained peaceful, deadly clashes erupted in the port city of Alexandria, where protesters set fire to the Brotherhood’s headquarters. Security officials said that among the victims was an American citizen, a man who was stabbed to death near the headquarters.

Early Saturday, officials of Kenyon College in Ohio identified the American who was killed as Andrew Pochter, 21, a Kenyon student from Chevy Chase, Md. United States Embassy officials said he died during clashes between supporters and opponents of Mr. Morsi. At least two other people were killed in the Alexandria fighting, including a 14-year-old boy, Ahmed el-Deeb, and Mahmoud Suleiman, a 26-year-old laborer who died in the hospital on Saturday after being struck with birdshot in his head, state media reported.

The violence on Friday provided a dark prelude to planned mass protests on Sunday by Mr. Morsi’s opponents, who are demanding that the president step down and early elections be held. Fears about possible violence at the marches have preoccupied the country for weeks and further split Egypt’s deeply polarized political class.