Several thousand Egyptians, joined by a prominent opposition leader, Mohamed ElBaradei, protested Friday what they said was the government’s systematic use of torture. It was the largest demonstration to date in response to the June 6 death of a young man in which the police are suspected.

The death of the man, Khaled Said, has become a rallying point for advocates of reform and human rights workers who say that police abuse is rampant and made possible by a three-decade-old emergency law.

“It’s a clear-cut message to the regime that the Egyptian people are sick and tired of practices that are inhumane,” Dr. ElBaradei told reporters after the protest. “If they don’t get the message, then there is a problem with the regime; the writing is on the wall.”

He described Mr. Said’s death as a “heinous crime.”

Witnesses say that police officers dragged Mr. Said, a 28-year-old businessman, out of an Internet cafe in Alexandria and beat him in the entrance of a nearby building.

The government maintains that Mr. Said was a criminal suspect who suffocated after swallowing a packet of drugs.