It’s an agonizing predicament: what to do when you see a pet in peril after being left in a scorching vehicle by its owner.

On Thursday, a new Massachusetts law took effect that provides a measure of protection for bystanders who step in to save an animal’s life.

Here’s how the law works: Bystanders are now allowed to break into a car to free an animal from extreme weather conditions if they decide it is reasonable to do so. They must first call 911 and wait with the animal until its owner returns.

Law enforcement and animal control officers are also allowed to break into a car to remove an animal if they leave a written record for the owner.

Advocates hailed the law, signed by Governor Charlie Baker in August, as a major step in preventing animal cruelty.

“The goal of this bill is prevention and trying to get animals out of these harmful situations before they get worse,’’ said Kara Holmquist, director of advocacy for the Massachusetts Society for the Prevention of Cruelty to Animals, which lobbied for passage of the law.

Also under the law, owners cannot leave their dogs tethered outside for more than 15 minutes when there is a weather warning, watch, or advisory in place, or if weather conditions could be harmful to the dog.

“It gives an additional enforcement ability to get at some of those harmful situations,’’ Holmquist said. “During the winter months, those animals don’t have adequate protection from the elements.’’

In July, a 2-year-old German shepherd named Maverick chewed off part of his own paw to escape a tether in Marlborough. He was rescued by an animal control officer, and was recovering after being adopted.

Olivia Quintana can be reached at olivia.quintana@globe.com. Follow her on Twitter @oliviasquintana.