Is it legal for a manager, in Califonia, to force his/her employees to work "mandatory" overtime?

For example: I have a 8-5, M-F, 40-hours-per-week job. My manager has been making us stay an extra 2 hours every day for the past 2 weeks. He also made us come in for a minimum of 4 hours last Saturday and is doing it again this Saturday. He is saying that it's all mandatory. Is that legal?

There isn't a restriction against an employer for requiring overtime. However, overtime must be paid to all non-exempt employees working over 8 hours in a day or 40 in a week (for California) with a few exceptions.