Can activated abilities on cards be used during an opponent's turn, and what timing restrictions apply?

Ruling: Activated abilities follow the same timing logic as spells. Abilities with "Reaction" or "Action" speed tags can be used during any appropriate timing window on any player's turn. Abilities with no speed tag can only be used on your own turn outside of combat.

Sequence:

• Check if the activated ability has a speed tag (Reaction or Action)
• If it has Reaction, it can be used during any reaction timing window on any turn
• If it has Action, it can be used during any action timing window (like during a showdown) on any turn
• If it has no speed tag, it can only be used on your own turn outside of combat

Nuances:

• Different abilities may have different speed tags, so not all abilities on different cards can be used at the same timing
• Action speed abilities like Malzahar can be used during showdowns on the opponent's turn when you have priority