Police Act 1964
Section 51 - Assaulting or Obstructing a Constable in the Execution of His Duty
Any person who assaults a constable in the execution of his duty, or a person assisting a constable in the execution of his duty, shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding six months or a fine not exceeding level 5 on the standard scale or both
Any person who resist or willfully obstructs a constable in the execution of his duty, or person assisting a constable in the execution of his duty, shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding one month or to a fine not exceeding level 3 on the standard scale or both
These offences are triable offences only.
One element common to both offences is that the conduct must take place when the police officer is acting 'in the execution of his duty'. The courts have constantly broadened the ambit of a constable's duty, but for our prepossesses it is likely to include a duty to prevent harm to persons and property, a duty to keep the peace and prevent aggravated trespass etc.
Hence if a constable reasonably anticipates a real possibility of a breach of the peace he is under a duty to take such reasonable steps as he thinks necessary to prevent the breach from occurring or continuing, and anyone who makes the task more difficult is obstructing the constable in the execution of his duty.
"Assaults or resists": an assault is either the application of force to the person of another, or the threat to apply it in such a way as to cause the person to fear or apprehend that he is about to be subjected to force.
Self defence against unlawful interference by a policeman is not an assault, however the difficulty is that once violence of any degree is resorted to it tends to become part of the escalating spiral
Meaning of obstruction:
"obstructing, for the present prepossess, means making it more difficult for the police to carry out their duties."
However, note that since Rice vs Conenolly (1966) it is established that a refusal to answer police questions is not an offence under S.51(3).
The power of arrest for obstruction, although the courts have persistently held there is no power of arrest for this offence, in practise as many of us have seen, is ignored with absolute impunity.