Without a context, I can't say for certain.
My first reaction to both sentences was about the same, and your option "b" is closest to the meaning I took for the sentences (except you should say, "They were going to...").
However, further consideration suggests more possibilities. Sentence 1 could be seen as defending their rights to go to the mayor's office, while sentence 2 could be talking about physically defending them from people trying to stop them getting to the mayor's office.
Context is so very important in determining fine points of meaning.