Can you activate a planeswalker loyalty ability at instant speed?

Loyalty abilities are a special category of activated abilities, but they come with strict timing restrictions. Under CR 606.3, a loyalty ability may only be activated any time you have priority during your own main phase, and only when the stack is empty. This is the definition of 'sorcery speed.'

Additionally, CR 606.4 states that a player may activate a loyalty ability of a permanent they control only once per turn. This means even if you somehow gained extra priority windows, you still get at most one activation per loyalty ability per turn (though you can activate different loyalty abilities if a planeswalker somehow had multiple).

This is different from most other activated abilities (like tapping a creature for mana), which can generally be used at instant speed unless otherwise restricted. Loyalty abilities have this restriction built into the rules themselves, not printed on the card.

Concrete example: You control Chandra, Torch of Defiance on your battlefield. Your opponent attacks you, and you want to use her +1 ability to deal 2 damage as a 'surprise.' You cannot — loyalty abilities can only be activated in your own main phase when the stack is empty, never during combat or in response to another spell or ability.

No card effect or rule override allows loyalty abilities to be used at instant speed unless a card explicitly grants that permission (such as a hypothetical effect saying 'You may activate loyalty abilities any time you could cast an instant').