LibreOffice: Make all references in selection absolute

Select the cells you want to change. Key Shift - F4 to cycle through the different forms of reference, A1 → $A$1 → A$1 → $A1 → A1. This changes all cells in the selected range.

This works with non-contiguous ranges too. Tested on LibreOffice 3.4.5 on Linux and 3.5 on Windows.

In LibreOffice 5.4.6.2, this can be done much simpler. Just select the Area and click Sheet → Cycle Cell Reference Types (Shortcut: F4)