This is not an easy question, and generally you have three choices, none of which are prefect but they are to be considered:
1. You put your employer on formal notice of having to work overtime and then you file a claim with DLSE for unpaid overtime. The obvious risk is that the employer will retaliate and fire you for filing that claim. Even though this would be illegal, your employer can still choose to terminate you and then deal with the legal consequences of that termination later.
2. You comply with your employer's instructions not to work overtime and leave after 8 hours of work, no matter how far behind you are on your work on that day. The risk here, of course, is being disciplined and fired for insufficient performance. Arguing that you weren't given an opportunity and enough time to finish work will not make this type of termination illegal, if you are an at will employee, because the employee is entitle to their subjective view and expectations of how they want their employees to perform.
3. You tolerate the situation for the time being and/or complaining to HR or higher management about the issue, and you ask them for help with your schedule and compensation. Again, there is a retaliation risk here, but it's probably lower than going straight to filing a wage claim with DLSE.
If you are facing this type of situation, you should consult with an experienced employment attorney to discuss which legal options is best of you, and whether there are other (creative) ways of handling it.