Your employer's might try to suggest that, by not going along with their plans, you've effectively tendered your resignation. Alternatively, you might feel forced to resign. If this happens you could well have a case for a claim against the company for 'constructive dismissal'. It's important to note, however, that a claim for constructive dismissal will normally fail unless the employee has initiated a formal grievance procedure prior to resigning (or being deemed to have resigned).
As it's possible that things could 'blow up' either tomorrow evening or on Saturday morning, you need to write a letter straight away and deliver to your employer first thing tomorrow morning.
The letter should be headed 'Formal Statement of Grievance' (so that there's no doubt about its purpose). Make sure you include the date. The letter should then state that you were engaged to work for the firm on the understanding that you would only work your existing contracted hours and, further, that you find the firm's new proposals to be both unreasonable and unacceptable.
Then include a paragraph stating "I have been advised that, should the company terminate my employment because of my refusal to work hours outside of my existing contract, I should seek redress, through the courts, for unfair dismissal. Further, I have also been advised that, should I find myself forced to resign my post through the unreasonable actions of the company, I should seek to bring a claim against the company for constructive dismissal"
None of that can stop the firm from sacking you but it might well be enough to make them back down.
Good luck!
Chris
PS: I know it's probably stating the obvious but don't forget to keep a copy of the letter.