Video - Police arrest protest pensioners
It appears that there was a protest relating to cuts to rail travel passes. Such passes are (or have been) very beneficial to many elderly people.
It is not fully possible to properly determine just from the video whether the force used by the Police was "reasonable" (as is required by the Criminal Law Act 1967 section 3 / Police and Criminal Evidence Act 1984 section 117) but it certainly has the appearance of being very heavy-handed on the part of the Police. See - CPS on Reasonable Force
The powers of arrest of a constable are in section 24 of the Police and Criminal Evidence Act 1984 (as amended by the Serious Organised Crime and Police Act 2005).
Where an individual is aggrieved, a civil action against the Police may be possible though this is a diffcult legal area of law and practice. Legal aid may sometimes be available though the rules are particularly complex - see Changes to civil legal aid - Practical guidance for the Bar. An alternative is the use of Police Complaints machinery.