Thomas Aquinas Commentary Lamentations 3:61-63

"Thou hast heard their reproach, O Jehovah, and all their devices against me, The lips of those that rose up against me, and their device against me all the day. Behold thou their sitting down, and their rising up; I am their song." — Lamentations 3:61-63 (ASV)

Here he denounces his enemies' sin.

1. Regarding the infliction of reproaches: you have heard their reproach, as if to say, “There is no need for proof.” Jerusalem, and your people are a reproach to all that are round about us (Daniel 9:16).
2. Regarding their threats: the lips of those who rise up. They that sought evils to me spoke vain things (Psalms 37:13).
3. Regarding their mockery and insults: their sitting down, because they sit in the assembly to ruin me; their rising up, in exaltation; and their song, because they make songs to mock me.

All this can be understood as being said from the perspective of the prophet or of the people, as was mentioned above. Now I am turned into their song, and am become their byword (Job 30:9). They that drank wine made me their song (Psalms 68:13).