If the lecturer teaches professionally, the students will shut up and listen. At the very first lecture, define the standards of behaviour expected and what they can expect from you. Explain when the lectures will start and how long you will speak for. Explain whether you would prefer people to put up their hand or just call out, and what they should do if they put up their hand and you don't see them. And if they don't, tell me and I will deal with them.
Punctuality is essential, especially with large groups.
A lecturer should arrive ahead of the scheduled time to set up the lecture aids and be present when the majority of the class arrives. One or two may be okay, but a larger group should be tackled so that the problem does not recur in later lectures. An astute lecturer will be aware when students are late due to a test or an assignment deadline imposed by other courses but this should not impinge on other lectures. One solution is to ask the class if they would like to start each lecture at 5 minutes past the hour. Invariably they will support you in starting sharp on the hour.