Protests are taking place nationwide today against a new housing benefit, labelled by campaigners as "bedroom tax", that will cut benefits to people with a spare room.
Under the Government's welfare reforms, those deemed to have a spare bedroom in their council or housing association home will have their benefit claims reduced by £40 to £80.
More top news
The man was taken early this morning to a south London police station where he remains in custody.
The Chancellor faces tough talks to build a Government while also dealing with the challenge from a resurgent far right.
Watch John McDonnell's speech at Labour Party conference 2017 live as he speaks to Brighton the audience from 12.15pm.