Why is 'Prevent' so controversial?

The National Union of Students (NUS) is being urged by ministers to support the government's counter-radicalisation programme, Prevent.

Universities Minister Jo Johnson has written to the NUS saying it is "disappointing" to see its opposition to the Prevent strategy.

BBC News looks at why the NUS is opposed to Prevent and why Prevent is seen by some as deeply controversial.

Producer: Ed Ram