After so many days of waiting, Egyptians finally have something to celebrate. But they must not do so yet: Mubarak's departure, seismic though it is, was never the final goal of this uprising.

The real aim is not the removal of one tinpot dictator, only for him to be replaced with an equally repressive system. The real goal is something much more fundamental: the removal of a corrupt and parasitic power structure and its replacement with transparent, open government and the rule of law.

For the moment, Suleiman remains integral to the state. So do many senior ministers. So do the senior military officers and the businesspeople whose vested interests count for so much in Cairo. This is a historic opportunity to sweep them all away. But now is the time, not tomorrow. This is the moment, and it is a brief one. If it is not seized, the loss will be momentous.

In the Philippines, in Ethiopia, across Eastern Europe, wherever lasting change was achieved, it was achieved because democratic voices made themselves heard at once. If that does not happen right away the danger is that Egypt will go back to business as usual.

So the critical question is "How do we give voice to the millions of newly politicised Egyptians?" They have no affiliation but they are determined to be heard. Unless they are organised their voice will fade away. They must immediately raise the critical questions for the future of their country. Who will be in charge of building newly open institutions? Will it be the military or Omar Suleiman? And can they possibly sacrifice their economic interests to more pluralistic, install democratic values – or will they fight to the end to preserve the status quo?

Those are troubling matters. Still, for all the dangers, no one should underestimate how powerful a moment this is. The departure of Hosni Mubarak marks the beginning of the fall of the authoritarian wall in the Arab world. Mubarak was the public face of political authoritarianism in the region: he has built one of the most feared security forces in the world numbering almost 1.5 million. His exit proves the power of the people. It removes the barrier of fear in the region. The events since 25 January have created a sense of empowerment that has swept Arab societies on every level.

In Egypt itself the victory has been sudden and only the night before it seemed a long way away. The old guard have been trying hard to find ways of keeping Mubarak in power. The Army and the Americans, perhaps, made the difference. It was the Army statement yesterday morning that made it clear they were telling the dictator "It's not working any more." And yet they have fought tooth-and-nail to keep the system in place, to keep the reforms incremental, to sustain the system under which they have risen so high.

Those efforts tell you just how removed from reality they are, how delusional, how clueless. Until the very last moment, I don't believe Mubarak had any intention of leaving: he and Omar Suleiman simply did not understand the gravity of this crisis. Instead, they treated a cancer as migraine.

But in the power structures that have been left behind, some vestiges of the sickness remain. Egypt will not truly be cured until the entirety of the old regime is gone.

The author is director of the Middle East centre at the London School of Economics