Mayor Bill de Blasio, facing an uproar over arrests at a Manhattan rally to protest strong-armed policing, offered a spirited defense of law enforcement on Thursday, brushing off concerns that demonstrators had been mistreated even as he insisted on his own commitment to reform.

Invoking his own past as a liberal organizer, Mr. de Blasio urged reporters “not to exaggerate what happened” at the Union Square rally on Wednesday night, saying the Police Department had acted appropriately in arresting 143 people marching to protest the death of a Baltimore black man, Freddie Gray, in police custody.

Two of those protesters were arrested after assaulting officers, one of whom was struck on the chin with a stick and injured, police officials said.

Police Commissioner William J. Bratton acknowledged that his department had staged an “assertive” response to keep demonstrators from blocking traffic in busy parts of Manhattan.