Undergrads at U of T's St. George, Mississauga and Scarborough campuses began to walk just after noon. About 3,000 students intend to participate in today's action, according to the students' union Facebook page.
Canadian Union of Public Employees Local 3902, which represents some 6,000 teaching assistants and other non-tenured staff, have been on strike for two weeks.
Classes have not been cancelled but some students say their education is being compromised.
"We're being told constantly that it's business as usual, and clearly it's not," student Liam Fox told CBC News. "Students are having courses cancelled, tutorials cancelled. They have no idea what's happening with their unmarked assignments, they have no idea when exams are going to happen.
"In a lot of cases, students don't know if they're going to graduate this year."