You can interrupt a speaker politely by saying "excuse me". You can also give a gesture by raising your hand a little bit. This is the polite way of interrupting someone. A slightly less polite way is to say "just a moment", which means the other person should stop talking so you can say something. "Peter Piper picked a pack of peppers..." "Excuse me, would you say that again please?" "Sure. Peter Piper picked a pack of peppers." "Football is the worst game in the world..." "Just a moment, I don't agree with that at all. I think it's the best game in the world!"

I would say something like: Excuse me. Would you repeat the last sentence over again please?

I the you the key is to respect polite manner, we let the speaker finish before speaking ourself. Then any word could be ok. I would say myself "sorry, could you repeat please". (I feel more polite to ask "could" rather than "would")