The union for faculty at 14 of Pennsylvania's state colleges said on Wednesday their strike would continue until they reach a deal in negotiations on pay and benefits, after professors walked off the job and left 100,000 students in the lurch.
Some 5,500 professors and other workers began the strike at 5 a.m. EDT (0900 GMT) on Wednesday.
Contract talks between the Association of Pennsylvania State College and University Faculties (APSCUF) and Pennsylvania's State System of Higher Education have been going on for two years.
The strike has affected about 100,000 students at schools from West Chester University near Philadelphia to California University outside Pittsburgh.
The APSCUF in a message on Twitter on Wednesday evening said the strike would continue on Thursday and last until "negotiators reach a contract."
"Speaking of a contract, we will let you know when negotiations resume," the union said in a follow-up tweet. "We are ready to return to the table!"
The union said it proposed a wage package smaller than those given to other state employees and offered healthcare concessions in excess of $50 million, which the school system rejected.
School system officials said they had offered a new proposal that included raises for permanent and temporary faculty and a healthcare package identical to what other state employees receive in order to avert a strike.
The walkout, which involves full-time and part-time faculty, comes amid a protracted struggle by graduate student workers across the United States to unionize and receive better pay.
Pennsylvania Governor Tom Wolf has urged both sides to return to the bargaining table.
The governor said he had increased funding to the state system by more than $30 million in the past two years to offset cuts made under the previous administration.
"Everyone's top priority should be the students and their families who are counting on an agreement to ensure Pennsylvania continues to deliver on its promise to provide a world-class college education," he said in a statement.
In a video posted on Twitter by West Chester University of Pennsylvania Professor Casey Bohrman, a couple dozen people could been seen picketing in front of a school building.
Students and alumni chimed in on social media, some showing support and others making light of the work stoppage.
"Bring back chips and pickles or we strike!!@BUCampusDining," Cecilia Nguyen, who identified herself as a Bloomsburg student, wrote.
(Reporting by Laila Kearney and Alex Dobuzinskis; Editing by Scott Malone, Tom Brown and Michael Perry)