This is another time to ask uncomfortable questions. Why are issues coming up in the middle of the iteration that "take higher priority"? Why can't they wait?

Is it a production bug? If it is, can it wait? Just because it's a production bug doesn't mean it must be addressed outside the normal iteration cycle. Criticality should matter.

If it's not a production bug...

Is it a missed critical requirement? WHY??? How did the entire team miss something that must be done right now? That's a major process failure.

If it's not a missed critical requirement...

Is it someone with great power just disrupting your iterations for their pet project? Do you have multiple customers who are competing for your team as a resource, and not hashing priorities out together in iteration planning?

Following from that...

Are they directly in your chain of command? Is it your superior changing horses in midstream, or your customer? Because you can tell customers no. Even if they scream and shout and say they're gonna tell Mom. The customer is not always right. Learn to say no.

And if you still feel like you have no authority and no control over your work...

Quit.