I hope there is a backup from before you started.
Mavericks and FMP 12 have reported issues.
Breaking the deletion into smaller chunks (20,000) would be my approach. Save As Smaller after each deletion.
I would not interrupt unless you have a known good backup.
i am working from a backup so its no big deal. i am half done now, but its been running for 6 hours now. ugh. i will just let it finish.
Turning Off all indexing on all fields in the table where this can be done might greatly speed up the delete. In other cases, it may work better to invert the process--save clone of your file to get a copy empty of all data and then import data into it instead of deleting data from a table.