A week after thousands demonstrated in support of Saied, the growing number of protesters on each side raises the possibility of Tunisia's political divisions spiraling into street confrontations between the two camps.
Police blocked the march without violence, as some demonstrators threw plastic bottles.
Saied dismissed the prime minister, suspended parliament and assumed executive authority in July in moves his foes call a coup.
Last month he brushed aside much of the constitution, which he said he would appoint a committee to amend, adding that he could rule by decree.
His intervention has cast into doubt the democratic gains made by Tunisians during a 2011 revolution that triggered the Arab Spring uprisings.