The strike by Tufts Medical Center nurses started at 7 a.m. Wednesday with picket lines outside the hospital.
Hospital officials have locked out striking nurses for four additional days and hired more than 320 temporary nurses.
The union and the hospital are at an impasse over staffing levels, pay and retirement benefits.
A union representative says hospital management was “determined to force a strike.” Hospital CEO Dr. Michael Wagner said it’s “extremely unfortunate that the union has continued to hold out for more money and an ill-conceived pension plan.”
The sides have been negotiating a new contract since April 2016.
Copyright 2017 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.