Offences Against Public Authority
It is an offence to assault, resist or wilfully obstruct police officers when acting ‘in the execution of their duties’, and similarly with public officers and persons lawfully making arrests (s34B, POA). In practice, charges generally relate to police officers. Before a charge can be made out it is essential to prove the police officer was acting in the execution of their duty and not exceeding that duty.
Obstruction of police is any act which makes it more difficult for the police to carry out their duty. This would generally be physical acts but it might include words e.g. urging a person in custody in a police car to get out. The obstruction must be intentional; an accidental or inadvertent act is not enough.
To resist arrest is to act so as to avoid being taken into custody. In order for the charge to be made out there must be some attempt by the police to effect a lawful arrest.