Much will depend on who is demonstrating, and how big the numbers are.

One can imagine a situation with big numbers, a broad coalition (including the PPP, some of the various Pakistan Muslim Leagues, the judges and lawyers, and--critically--ex-military types) that local security forces on the ground simply refuse to confront. If so, it then could go downhill rapidly for Musharraf.

I'm far from a Pakistan expert (at all), but I suspect we see 5 scenarios at this point:

1) Musharraf rides out the initial storm of opposition, consolidates, and filled with new purpose and a strengthened position goes after radical islamist opponents with new effectiveness. Odds: very low.

2) Musharraf hangs on to power with badly damaged legitimacy. Elements of the army waver, are disloyal, or are simply poorly motivated. Radical recruitment and influence bolstered. Odds: moderate

3) Opposition grows and broadens at such a rate that elements in the military refuse to support Musharraf. Caretaker government pledges elections, in which elements of broad opposition coalition maintain cooperation, resulting in strengthened, redemocratized central government. Odds: very low.

4) Opposition grows and broadens at such a rate that elements in the military refuse to support Musharraf. Replaced by another military figure. No rapid transition to democracy; instead new military regime attempts bargains with PPP or others to broaden support base. Looks like the situation a year or two ago. Odds: low-medium

5) Opposition grows and broadens at such a rate that elements in the military refuse to support Musharraf. Caretaker government pledges elections. Opposition fragments, elections are controversial and the results indecisive. Political infighting abounds, sapping political and governmental capacity. Odds: moderate.

The odds of #4 increase if radical Islamist groups engage in spectacular violence in the next few weeks, alarming senior military leaders about the viability and effectiveness of a Musharraf government.

Anyone more knowledgeable than I able to highlight what I may have missed?