I think the second option should be the correct one since fifty per cent is plural and fewer is used with countables."Of the people" is only a prepositional attribute and as such simply part of the subject but not what I should call the subject proper with which the predicate is supposed to agree.
Again, which version is to be preferred:
A large percentage of the population were against nuclear weapons
A large percentage of the population was against nuclear weapons?
I look forward to your answering my questions. Please tell me also whether I put my questions correctly and understandably.