Good advice given so far, but if you are a member of a union it is absolutely vital that you tell them BEFORE you go to your line manager (or their superior if its the line manager harrassing you).
I've seen unit managers make all sorts of nice noises about how they'll deal with the problem and do nothing, either because they don't believe you, can't be bothered, don't know how to deal with it, or, worst of all, if the bully is someone the manager thinks highly of, they somehow want to protect the image of the bully and don't want anything to happen which may tarnish the manager's image.
Get the union involved immediately, it is highly likely that the unit rep' will pass it on immediately to the full time official for them to deal with.
Even if you are not a member of the union, talk to any local rep' - any union representitive worth their salt will give you confidential advice.
Whatever you do, take the advice given so far and act now.
Good luck.