Running battles between supporters and opponents of deposed Egyptian president Mohamed Morsi near Cairo's Tahrir Square on Monday left one person dead and more than 20 others hurt.
Security forces fired tear gas to try to disperse the crowd, Reuters cited witnesses as saying.
CNN wrote that at least some of the clashes occurred when Morsi supporters conducting a planned march on the US Embassy changed direction after being chased away by Egyptian security forces.
They then moved toward Tahrir Square, less than a mile away, where Morsi opponents had gathered.
Reuters cited a bloodied protesters, Samir Hafiz, 22, as saying:
"The police are with us. Those fascists attacked us again. May God punish them."
The clashes come nearly three weeks after the Muslim Brotherhood-backed Morsi was ousted from the presidency by the Egyptian military.
He has not been seen or heard from since, and his family on Monday accused the army of "kidnapping" him.
It is the worst violence in the Egyptian capital since July 16, when seven died in confrontations.