TORONTO -- Workers who provide technical and staging expertise for events in major venues at Toronto's Exhibition Place have been locked out.

International Alliance of Theatrical Stage Employees Local 58 says the lockout comes after months of negotiations for a new collective agreement.

Exhibition Place says it locked the workers out, effective Friday morning, following negotiations that included meetings with the assistance of a provincially appointed mediator.

The union says Exhibition Place had asked it to suspend picketing until after the Canadian Nation Exhibition -- which begins in mid-August -- closes on Sept. 3, but local president Justin Antheunis says "that's not going to happen."

The affected venues include BMO Field, Queen Elizabeth Theatre, Coca Cola Coliseum, Enercare Centre and the Liberty Grand, and the union said picket lines would go up Friday afternoon.

Exhibition Place says it has a contingency plan in place and remains open for business.

"The safety of all events at Exhibition Place will be compromised by companies bringing in less skilled workers to set up and supervise events," the union said in a release.

Both sides said they would be willing to return to negotiations.