Align Objects > Centered, Page Both (Modify Menu)

This command can be used to center all kinds of objects horizontally and vertically on the page. They are moved horizontally so that they are the same distance from the left and right edges and the top and bottom of the page. All chosen objects are treated as a single item, i.e. they are all moved by the same amount.

All the objects to be centered have to be chosen. Click on objects to choose them.

The orientation is determined by the overall bounding box of all objects, not by their actual contours.