The most significant difference between assault on a police officer and a normal assault case is that a person is not legally permitted to use force to resist an arrest even if the arrest is ultimately found to be unlawful. Even if a person can prove that an arrest or a police action is not legally justified, a person in that situation is still not able to use force to resist that arrest.
However, a person is allowed to resist an arrest when the officer uses substantially more force than necessary under the circumstances.