One day after a grand jury declined to indict a New York police officer in the death of Eric Garner, prompting angry protests and calls for reform from elected officials, Mayor Bill de Blasio on Thursday announced the start of a significant retraining of the nation’s largest police force.

Mr. de Blasio said the grand jury’s decision had focused the public’s attention on the relationship between the police and the public.

“Fundamental questions are being asked, and rightfully so,” he said. “The way we go about policing has to change.”

The de Blasio administration did not immediately explain in detail what the training would entail and how the city would cover the considerable costs of such an undertaking.