I can add couple of points to the above comments:
Read your contract again, some contracts will state that you need to work reasonable hours (above the weekly hours) to have the work completed (as most of my contracts), this is from the legal aspect.
other point, I like to mention, is that the economy is not good now and it maybe worth while to work this to be have a good environment in the work place, complaining or refusing this may harm your future with the company, also: - is it a small business?,- have you had any other indications or examples that have a negative vibe? if there are none, it could be the the manager is trying to run the business better.
Hope this helps and gives you things to consider,
good luck.