Shia students in Najaf, 160km south of Baghdad, marched towards the tomb of Ali, chanting: "The people will revolt against America and the constitution."
"Yes, yes to unity, yes, yes to the Marjaya (the Shia religious authority)," the students roared.
The crowd included supporters of both popular cleric Muqtada al-Sadr and the most revered Shia figure, Ayat Allah Ali al-Sistani.
Al-Sadr is an outspoken opponent of the interim constitution and has said a new document should be written, based on Islam.
Al-Sistani has expressed reservations about the document, but stopped short of demanding mass demonstrations over the it.
In Mosul, 370km north of Baghdad, Arab students, both Shia and Sunni, demonstrated against the interim constitution. "No, no to the constitution. Yes, yes to national unity, yes, yes to Islam, no, no to occupation," the protesters said.