What's the datasheet say? Cover page of Summary data sheet shows this:
– Internal 115KHz (RCSYS) and 8MHz/1MHz (RC8M) RC Oscillators
Section 1 says:
The CPU runs from the on-chip RC oscillators, the PLLs, or the Multipurpose Oscillators.
The AT32UC3C is a complete System-On-Chip microcontroller based on the AVR32UC RISC processor running at frequencies up to 66 MHz.
Reading section 7 of the main datasheet, looks like it should be okay with the internal clock:
22.214.171.124 Selecting the main clock source
The common main clock can be connected to RCSYS or a selection of other clock sources. For
details about the other main clock sources, please refer to the MCCTRL register description. By
default, the main clock will be connected to RCSYS. The user can connect the main clock to
another source by writing to the Main Clock Select (MCCTRL.MCSEL) field. The user must first
assure that the source is enabled and ready in order to avoid a deadlock. Care should also be
taken so that the new synchronous clock frequencies do not exceed the maximum frequency for
each clock domain.