I've done it both ways and think both are acceptable.
Either make a loud general announcement "Food is ready. Please head over to the dining room and help yourselves." -Or- do a general mingle where you join into a group having a conversation and just interject "does anyone need a drink refill?"
I tend to have a lot of parties so most of my friends know where the food and drink are and know to help themselves. In addition, I usually give instructions as I greet people at the door. So I might tell them where to put their purse or coat. If they've brought food (for a potluck) I'll direct them to the table or dessert area, etc. And while I'm doing that I usually say, "drinks are in the kitchen, please help yourself".
But for example, I had a baby shower for a friend recently which included several guests who'd never been to my house before. I made a point of directing them to the drinks if I noticed they didn't have one because although I might have stated at the door, it might not have sunk in, what with saying hello & nice to meet you & such. Then, when it was time to eat, I made a general announcement. Yes, conversations were interrupted but, hey, it's food! No one seems to mind.
I still had to goad some people into going on over to the dining room to get food so I sort of interrupted those groups individually.
I agree with PPs who said, it's not rude to interrupt conversations with hosting duties. It happens. It's part of being a host. For example, if you were having a seated dinner with cocktails beforehand, you would necessarily have to interrupt conversations to announce that dinner was served so that everyone would head over to be seated at the table.