atmeg328p at low voltage / frequency

If I want to run an atmega328p at lower voltage, say 2.4V, then I need to use a slower clock.

The question is: will dividing the internal clock (with fuse settings) do, or not?

Does the internal clock require a certain voltage to maintain its 8 MHz, regardless of the divisor, and do I consequently need external crystal to run at for example 1 MHz and lower voltage?

|500x263

More details in the datasheet.