CAIRO — The first major protest aimed squarely at Egypt’s transitional military rulers ended Saturday night in violent clashes with neighborhood youths, who are opposed to the continued demonstrations that threaten stability.

Egyptian Health Department officials said at least 25 people were hospitalized and 120 others were treated on the scene.

The demonstration, on the day Egyptians celebrate the 1952 military coup against the British-backed monarchy, underscored how liberal elements of the protest movement have increasingly turned on the army after the heady days of the demonstrations that forced President Hosni Mubarak from power.

But it also showcased both the mounting hostility toward the Tahrir Square activists in some precincts more worried about stability and economic growth than swift political change. And, following several other recent bursts of nocturnal street violence, it was the latest reminder that the potential for chaos is still present in this hot, overcrowded city where the still-despised police force has yet to reconstitute and reassert itself.