As head of organizations during my former years. I faced the same problem. I just stood at the podium until everybody realized the meeting was not going forward and they quited down. There were times, (I timed the lapses) when I would have to wait 5 minutes. They soon learned that nothing was going to be done until they "shut up".
In addition to this, I play bingo 2 nights a week. People love to gab. When the caller is ready and they are still gabbing, I whistle. It become quiet very quickly.