Protests in Egypt tend to follow a familiar rhythm. As a veteran observer of more public demonstrations than I can remember, you get a sense of the routine.

There’s usually a few hundred activists, most of them familiar faces from the last eight protests, inevitably surrounded by twice as many black-clad Central Security riot cops. The activists chant their slogans, the police use overwhelming force and well-practised crowd control techniques to keep them penned in one spot, and eventually everybody goes home.

But the protest I covered on Friday in Alexandria felt different and not just because it was attended by Mohamed ElBaradei, the former UN nuclear watchdog who has become an opposition figurehead. The case of Khaled Saeed – a young man beaten to death in public by police, according to multiple witnesses — has tapped into long-simmering tensions that could take Egypt into uncharted territory.

For starters, there was the sheer size of the protest – at least 3,000 people according to organisers. Beyond that, what was striking was the raw anger on display and the number of ordinary, normally apolitical citizens who turned out to protest against what they claim is endemic brutality among Egypt’s police and security forces.

Saeed, 28, was dragged out of an Alexandria internet café on June 6 by two plain-clothed police officers. Several witnesses and the café’s owner have given interviews saying they saw the officers brutally beat Saeed in an alleyway. He was pronounced dead on the scene.

Public anger spiked when pictures of Saeed’s badly mangled face circulated on the internet. Protests in multiple cities intensified when the Interior Ministry claimed the young man had choked to death when swallowing a packet of marijuana as police approached him. The ministry’s version of events was backed by two separate coroner’s reports, prompting claims of a coordinated cover-up.

Saeed’s case is hardly the first publicised incident of Egyptian police brutality. Local and international human right organizations have long documented what they claim is a systemic pattern of torture and intimidation in Egypt’s police stations. But this case has touched a deep and powerful nerve, resonating among ordinary citizens who had probably never considered attending a demonstration before.

When I pulled out my notebook at Friday’s protest, I was engulfed by people clamouring to tell me their own personal tales of injustice and mistreatment at the hands of the police. I could have written down a dozen examples, ranging from harassment and intimidation of political activists to Mafia-style shakedowns.

The framework for all this police misbehaviour is the Emergency Law — a regularly renewed piece of legislation which has placed Egypt under defacto martial law for President Hosni Mubarak’s entire 29-year reign. Saeed has already come to be known as the “Emergency Law Martyr” and activists are hoping to channel the current explosion of popular anger into a genuine push to finally get the law repealed.