The police commonly use this power to arrest protestors and demonstrators who are standing outside building, sitting down blockading entrances or roads and in many public order situations.
You may be committing this offence if you wilfully obstruct the free passage of the highway and when doing so you don't have a lawful authority or excuse.
The 'highway' is not just limited to roads but also includes pavements, grass verges and private property used as a public thoroughfare. Also, all members of the public (not just a portion of it) should have a right to pass, at will, along the highway for it to actually be a highway. The right to pass at will may be for a limited reason (i.e vehicles on a road, or pedestrians on a footpath).
‘Obstruction’ includes anything that prevents passing and re-passing along the highway. To be committing this offence, you don't have to be blocking the whole width of the highway. This is because the offence is obstructing the highway itself (and not other users of the highway). The prosecution, therefore, do not have to prove that anyone was actually obstructed, but instead that you obstructed the highway itself.
It must also be proved that the obstruction was 'wilful'. Commonly, the police will ask to to move and will often give you warnings to move. The police will normally be recording them giving you these warnings, as this could be used to prove that the obstruction you caused was wilful.
If you are found guilty of this offence in court, the maximum penalty is a fine of £1000. First time offenders are be likely to receive about £200 or a conditional discharge.