UPDATE for v1.1:
Rather than giving q="search_string" give it q="hashtag" in URL encoded form to return results with HASHTAG ONLY. So your query would become:
GET https://api.twitter.com/1.1/search/tweets.json?q=%23freebandnames
%23 is URL encoded form of #. Try the link out in your browser and it should work.
You can optimize the query by adding since_id and max_id parameters detailed here. Hope this helps !
Note: Search API is now a OAUTH authenticated call, so please include your access_tokens to the above call
Updated
Twitter Search doc link:
https://developer.twitter.com/en/docs/tweets/search/api-reference/get-search-tweets.html