The largest security training exercise in Boston’s history

is slated to kick off at 8 a.m. Saturday with a simulated active school shooter drill in Brookline. It will continue with other drills during 24 hours and involve about 2,000 state and local personnel.

“The Urban Shield Program, as we’ve learned over the last year, is essential to our city’s safety at any time of crisis,” Mayor Martin J. Walsh of Boston.

“Since the 2013 Boston Marathon, we learned a lot about resiliency as a community and about professionalism of our first responders. This initiative has a lot to do with that,” Walsh said. Residents should not be alarmed, authorities said, if they see an increased police presence Saturday.