Multiple national officials criticized the decision of the police in Ferguson to use military-style garb and equipment to respond to the protests. “At a time when we must seek to rebuild trust between law enforcement and the local community, I am deeply concerned that the deployment of military equipment and vehicles sends a conflicting message,” Attorney General Eric H. Holder Jr. said in a statement. Later on Thursday, Mr. Holder called Mr. Brown’s parents and promised a full, independent investigation, according to a Justice Department official.

Across the political spectrum, officials seemed to agree. “The militarization of the response became more of the problem than any solution,” Senator Claire McCaskill, Democrat of Missouri, told reporters in Ferguson. Senator Elizabeth Warren, a Massachusetts Democrat who has become a favorite of the American left, said on Twitter, “This is America, not a war zone.” And Senator Rand Paul, a Kentucky Republican with libertarian leanings and presidential aspirations, wrote an essay for Time in which he called the militarization of the police “an unprecedented expansion of government power” and said, “The images and scenes we continue to see in Ferguson resemble war more than traditional police action.”

Elsewhere in the country, rallies were held to demand justice for Mr. Brown and to protest police tactics. In Chicago, Los Angeles, New York and Phoenix, thousands of protesters gathered in public squares with their hands up in gestures of surrender, chanting slogans like, “Hands up, don’t shoot.” In Miami, eight people were arrested.

In Ferguson, officials were unapologetic on Thursday for their tough response to the protesters, which they said had been necessitated by violence and criminality: The police said some protesters had thrown rocks, bottles and even a firebomb at officers. If the situation warrants it, “the tactical units will be out there,” said the Ferguson police chief, Thomas Jackson. “If the crowd is being violent and you don’t want to be violent, get out of the crowd.”