A constable may arrest without a warrant—
anyone who is about to commit an offence;
anyone who is in the act of committing an offence;
anyone whom he has reasonable grounds for suspecting to be about to commit an offence;
anyone whom he has reasonable grounds for suspecting to be committing an offence.
If a constable has reasonable grounds for suspecting that an offence has been committed, he may arrest without a warrant anyone whom he has reasonable grounds to suspect of being guilty of it.
If an offence has been committed, a constable may arrest without a warrant—
anyone who is guilty of the offence;
anyone whom he has reasonable grounds for suspecting to be guilty of it.
But the power of summary arrest conferred by subsection (1), (2) or (3) is exercisable only if the constable has reasonable grounds for believing that for any of the reasons mentioned in subsection (5) it is necessary to arrest the person in question.
The reasons are—
to enable the name of the person in question to be ascertained (in the case where the constable does not know, and cannot readily ascertain, the person's name, or has reasonable grounds for doubting whether a name given by the person as his name is his real name);
correspondingly as regards the person's address;
to prevent the person in question—
causing physical injury to himself or any other person;
suffering physical injury;
causing loss of or damage to property;
committing an offence against public decency (subject to subsection (6)); or
causing an unlawful obstruction of the highway;
to protect a child or other vulnerable person from the person in question;
to allow the prompt and effective investigation of the offence or of the conduct of the person in question;
to prevent any prosecution for the offence from being hindered by the disappearance of the person in question.
Subsection (5)(c)(iv) applies only where members of the public going about their normal business cannot reasonably be expected to avoid the person in question.