TORONTO - Faculty at 24 Ontario colleges went on strike late Sunday, affecting more than 500,000 students.

The Ontario Public Services Employees Union says the two sides couldn't resolve their differences by a strike deadline of 12:01 a.m. Monday.

“There was really nothing left that we could put forward, nothing more coming from the employer,” Nicole Zwiers, a member of the union bargaining team.

The faculty regrets the effect on students, but many understand the issues at play, said Zwiers.

“It's always a case that there is always a high degree of upset, which is absolutely understandable,” said Zwiers in an interview Sunday night. “I think that many of our students are indicating to us that they understand the issues that we're facing.”

There was no indication on when talks might resume said Zwiers, but the union remained optimistic.

The College Employer Council, which bargains for the colleges, called the strike completely unnecessary.

“We should have had a deal based on our final offer. It is comparable to, or better than, recent public-sector settlements with teachers, college support staff, hospital professionals, and Ontario public servants - most of which were negotiated by OPSEU,” said Sonia Del Missier, a spokeswoman for the council, in a statement.

The union's demands would have added more than $250 million in annual costs, the council said.

The union presented a proposal Saturday night that called for the number of full time faculty to match the number of faculty members on contract.

It also called for improvements in job security and for faculty to have a stronger voice in academic decision making.

The strike involves more than 12,000 professors, instructors, counsellors, and librarians.