Toronto's library workers have walked off the job after their union and the library board failed to reach a labour agreement Sunday, despite extending the strike deadline several times.

About 2,400 library workers will begin picketing on Monday, CUPE Local 4948 president Maureen O'Reilly told reporters after the final deadline had passed.

"Despite our best attempts and numerous extensions to the deadline, negotiations have stalled," she said. "We find that we are left with little choice but to take job action."

Striking workers are also planning to rally in front of city hall on Monday.

All 98 library branches will be closed Monday. The library is asking borrowers to hold on to the books and materials they've checked out. No overdue fines will be charged during the strike.

"We remain committed to working with Toronto Public Library Workers' Union Local 4948 to reach a settlement that is fair and reasonable to our staff and affordable to the residents of Toronto so that we can resume public library service in Toronto as soon as possible," Coun. Paul Ainslie, chair of the Toronto Public Library Board, said in a news release.

Ahead of the original 12:01 a.m. Sunday strike deadline, O'Reilly had signalled that negotiations were not going well. She said CUPE's bargaining team was not happy with the offers the library board was putting on the table.

The two sides agreed to continue talks into the wee hours and remained at the table throughout Sunday.

But at 5 p.m., O'Reilly emerged from the Westin Prince Hotel where the marathon talks were held and said the union felt it had no choice but to initiate job action.

"Our members joined the public and fought library closures, but we still lost 107 staff in the city's 2012 budget," she said.

"The Toronto Public Library has been cut to the bone and these cuts are felt most deeply by the workers who are now being stretched to provide the same service with so much less."

O'Reilly said the union was only seeking job protection, not "major gains."

CUPE Local 4948 has accused the library board of making unrealistic promises to the city earlier this year when it agreed to budget cutbacks.

In January, Mayor Rob Ford's budget was passed with a 10 per cent funding cut to libraries. Ford initially wanted to slash the library budget by $7 million, but council later agreed to cut only $3.9 million.

O'Reilly said the library has cut 107 full-time jobs and only 22 per cent of part-time workers -- most of them women -- get benefits.

Librarians have been working without a contract since Jan. 1, as part of greater labour negotiations with other indoor workers in Toronto.