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.

Mr. de Blasio, a Democrat, convened a news conference after hearing complaints about the police response from allies like the Rev. Al Sharpton. “I’ve participated in plenty of protests, on plenty of issues,” the mayor said. “I believe deeply in how nonviolent protest has achieved social change.”

But, he added: “When the police give you instruction, you follow the instruction. It’s not debatable.”

The arrests on Wednesday, which prompted a protest from liberal groups outside Police Headquarters in Lower Manhattan, came amid a renewed debate in New York over aggressive police tactics, including an effort by the City Council to decriminalize low-level offenses, like public urination.

But Mr. de Blasio’s attitude was a far cry from late last year, when he allowed similar demonstrations to spill into the city’s highways and avenues. That episode, after a Staten Island grand jury declined to indict an officer in the death of a Eric Garner, an unarmed black man, contributed to an open law enforcement rebellion against the mayor.

On Thursday, as in the past on police issues, Mr. de Blasio seemed to struggle to walk a tightrope. Even as Mr. Bratton, in a separate news conference, acknowledged a “much more assertive” approach to handling street protests, the mayor insisted that “the strategic approach is exactly the same.”

Police said 143 people were taken into custody Wednesday night. Most were given tickets and sent home after ignoring repeated demands to not block traffic.