(AP) -- Hundreds of people marched through the streets of midtown Manhattan on Thursday to protest the recent police-related shootings of two black men.

The protesters first gathered in Union Square Park where they chanted "The people united, never be divided!" and "What do we want? Justice. When do we want it? Now!"

The protesters then left the park and began marching up Fifth Avenue as police scrambled to keep up.

Michael Houston, a 20-year-old Brooklyn student, said anger and lack of action brought him to the protest.

"It's the definition of insanity," Houston said. "How can we expect anything to be different when nothing changes."

Lawrence Amsterdam, 35, another student from Brooklyn, decried what he called the police injustice.

"It's supposed to be innocent until proven guilty. But the way I see it, it's murder first and ask questions later," Amsterdam said.

On Wednesday, a Minnesota officer fatally shot Philando Castile while he was in a car with a woman and a child in a St. Paul suburb. The aftermath of the shooting was purportedly livestreamed in a widely shared Facebook video.

A day earlier, Alton Sterling was shot in Louisiana after being pinned to the pavement by two white officers. That, too, was captured on a cellphone video.