The officer, 25-year-old Brian Moore, was shot in the head after stopping the assailant, who was "adjusting an object in his waistband," New York Police say.
Tony Robinson, 19, was killed March 6 by an officer. Police say he was shot after a confrontation in which he allegedly assaulted the officer. Protesters say the death is more evidence of racial bias.
Authorities say an officer was assaulted after responding to a call about reckless driving. The Madison police chief has acknowledged that the suspect was unarmed.
At least 49 were arrested Monday afternoon and into the evening for acts of civil disobedience during protests over the August police shooting death of 18-year-old Michael Brown.
The latest unrest comes after an off-duty police officer killed 18-year-old Vonderrit D. Myers, who authorities say fired three times in the encounter.
Thomas Jackson said he was sorry to the victim's family and to anyone who felt their right to "peacefully protest" had not been protected by authorities.
The Department of Justice says it will begin a review of police department procedures in Beavercreek, Ohio, where officers fatally shot a 22-year-old black man at a Wal-Mart store.