Under ordinary specific circumstances you can make a citizen's arrest to prevent loss/damage to property public or private or perhaps preempt a crime.
In organized society where public officials are bestowed clear definite powers to act they will have the full protection of the Law which of course is sanctioned by Parliament. Different than an ordinary citizen, a public institution is organized bureaucracy where instruments of service are enormous by duly and legally sanctioned actions and procedures.
OCCUPY sentiments elsewhere
What's most irksome is the fact that DBKL Officers stepped off their bounds of duty and arrested the persons of the demonstrators. The Police didn't, they couldn't as they had seemed to merge with the brickworks.
Unless duly conferred by an act of Parliament or a statute, no DBKL personnel should arrest a citizen. They can confiscate property deemed inapproprite ...they cannot confiscate citizens, people, human beings.
The Occuupiers could take legal action in this. They could sue the pants off the atrocious DBKL officers.
If most of the Occupiers are students they could learn the living now. They could later live the learning.