I guess, you can tell them "thanks for the offer!, but I need some time to decide, i'll probably let you know my decision in 3-4 days time".
If they press you about whether you have any competing offers/interview, then I think you can be honest about the second interview, and tell them that you have a final round of interview at the second place, and you'll be only able to make a decision after that interview is over.
Be Confident, and think that the you have the right to choose the best employer!
hope this helps!!
Good luck.