The best way to handle the problem is to interrupt the person who has interrupted you. Then, in a calm voice, you should ask, "Please wait until I finish my thought." In group discussions, the perpetrator of the bad habit is likely to back down.
Once you have completed your remarks, you might say to the individual, "Now, what did you want to contribute?"
If your colleague interrupts you even in one-on-one conversations, you may want to raise the problem with him or her in private. It could put an end to a good office friendship, and your colleague should be alerted to how annoying it is to be interrupted regularly.