Protesters argue with police while they take part in a protest against the killings of Alton Sterling and Philando Castile during a march in New York July 7, 2016.[Photo/Agencies]

In Chicago, protesters shut down a stretch of the Dan Ryan Expressway - one of Chicago's main arteries - for about 10 minutes on Thursday.

In New York, several hundred protesters blocked traffic in Times Square in the heart of Manhattan, chanting "Hands up, don't shoot." More than a dozen arrests were made, the New York Police Department said.

In St. Paul, about a thousand people gathered outside the governor's mansion, chanting "Hey hey, ho ho, those killer cops have got to go," and other slogans.

Minnesota Governor Mark Dayton made a brief appearance in an attempt to quell the crowd. He said earlier a state investigation was already under way.

State investigators later identified Minneapolis area police officer Jeronimo Yanez as the patrolman who fatally shot Castile during a traffic stop.

US President Barack Obama described the killings as tragedies.

The Washington Post said Castile was at least the 506th person and 123rd black American shot and killed by police so far in 2016, according to a database it has set up to track such deaths.