Staff from the University of Birmingham are taking strike action in a dispute over the living wage.
Unison said caterers, cleaners, security guards and other support staff walked out on Tuesday following "failed negotiations over fair pay".
It is the first of two consecutive days of action by union members, falling on the university's graduation days.
The university said it was "business as usual" on campus and that it has matched the voluntary living wage.
Unison said it expected about 300 people to take part in the the industrial action, which was voted for by more than three quarters of its membership at the university.
It followed a previous strike at an open day on 28 June.
On Friday, it said, members had rejected an offer by the university to pay a lump sum of 1% to all support staff.
Unison said it wants the university to grant a pay increase for all support staff; become a Living Wage Accredited employer; eliminate the gender pay gap by 2020 and negotiate with all unions on a joint report on working conditions at the university.
"We already know that members have turned to food banks to feed their families," it said.
"Many low-paid staff only work 15 hours per week and often hold several jobs to make ends meet."
The university said this was the tenth year support staff pay awards have been in line with, or above those negotiated nationally and the fourth consecutive year it has matched the voluntary living wage.
"We would like to thank all those staff who have shown their dedication to celebrating the achievements of our students and to making graduations continue as normal," it said.
There's such a fabulous buzz on campus as we're in the middle of graduations!— Uni of Birmingham (@unibirmingham) July 16, 2019
We love seeing all the families involved and the proud messages they're sharing on social media 😍
Find lots & LOTS of graduation images on @birminghamalum's Facebook page https://t.co/CwZV34NcAn pic.twitter.com/vB6DvWGsGV
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