Is it illegal to film police officers performing their duties in a public place?

Generally if public officers are in a public area and it is considered news worthy and in the public interest, you can film them (within reason).

No. You can record people in public places. However, if your presence increases the danger of a situation or to another person, a police officer can have you leave. If a police officer confiscates your phone, you may have a legal issue. This is currently a contentious area of law and several cases against police departments are on-going. Just keep Googling for the latest! If your phone is taken, please do see a civil rights attorney immediately.

No, as long as they are performing the duties they were hired to do and the place is indeed public.

CALIFORNIA: It is LEGAL to film / video police performing their duties in a public place. Police often make people move back. Police sometimes try to make people move back farther than really seems needed for safety. You have the right to film/ video, so carry that cell phone camera, and whenever you see something going on, turn it on and video.