Although the police have considerable powers, and although law-abiding citizens normally co-operate with the police whenever they are asked, the police do not have unlimited powers to demand such cooperation.
Rice v Connolly  2 All ER 649, QBD
A man Rice was seen by a police officer who thought he was acting suspiciously and asked for his name and address. Rice refused, and also refused to "accompany the officer to the police station", and was subsequently convicted of obstructing a constable in the execution of his duty. Quashing his conviction, Lord Parker CJ said that although every citizen might have a moral or social duty to assist the police, there is no legal duty to do so, and the whole basis of the common law is the individual's right to refuse to answer questions put by those in authority or (short of arrest) to accompany them anywhere.
Collins v Wilcock  3 All ER 374, QBD
A policewoman PC Collins wanting to question a woman Wilcock in the street took hold of her arm; Wilcock resisted and was charged with assaulting a constable in the execution of her duty. The High Court said since PC Collins did not claim to have been exercising a specific power to "stop and search", her taking Wilcock's arm was itself an assault, taking her outside the execution of her duty and entitling Wilcock to use reasonable force in self-defence. No assault is committed if a person merely touches another's arm or shoulder (with minimum force) to gain his attention, even more than once, but it may become an assault if a mere touch is replaced by a physical restraint.
R (Laporte) v Chief Constable of Gloucestershire (2004) Times 26/2/04, QBD
Coaches carrying demonstrators were stopped while travelling to RAF Fairford in Gloucestershire, where they planned to demonstrate against the war in Iraq. The police refused to allow the coaches to proceed, and insisted that they return to London under police escort; this took about 2½ hours, during which time the passengers were not allowed to disembark. May LJ said the police had reasonably feared a breach of the peace and had thus acted lawfully in stopping the demonstration, but the further detention of the demonstrators (in their coaches) for so long a period was disproportionate and thus unlawful.
However, it is an offence to wilfully obstruct a police officer in the execution of his duty.
Lewis v Cox  1 QB 509, QBD
A man was arrested for being drunk and disorderly and placed in the back of a police vehicle; his friend Cox opened the door to ask the man where he was being taken, but the police officer PC Lewis closed the door before the man could reply and told Cox he would also be arrested too if he opened it again. As PC Lewis returned to the front seat to drive away Cox did open the door again, and he was subsequently charged with obstructing a police officer in the execution of his duty. The Bristol magistrates acquitted on the basis that Cox had not committed any deliberate aggressive act aimed at PC Lewis, but the prosecutor appealed by way of case stated. Allowing the appeal and remitting the case with a direction to convict, the High Court said it was sufficient that Cox's act had in fact prevented Lewis from carrying out his duty, and that Cox had known it was likely to have that effect; his motive was irrelevant unless (which was not the case here) it amounted to a lawful excuse.
Sekfali v DPP  EWHC 894 (Admin)
Three men suspected of shoplifting were approached in the street by plain-clothes police officers; the officers identified themselves and the men immediately ran away in different directions. They were chased and caught, and were convicted by magistrates of obstructing a police officer in the execution of his duty. They appealed on the basis that (not having been arrested) they were entitled to run away, but the High Court dismissed their appeal. They could legitimately have remained silent or refused to answer questions, and could have refused to accompany the officers to the police station unless arrested, but running away was indeed an obstruction.