By Alta Spells and Eric Levenson, CNN

(CNN) -- Protesters demonstrated outside the Ferguson Police Department on Sunday night to mark the 6th anniversary of the police killing of Michael Brown.

Ferguson Police said over 100 people gathered at the department, and police ultimately arrested four protesters for failure to disperse.

Brown, an unarmed Black 18-year-old, was shot and killed in the street in 2014 by a White police officer, who said he shot in self-defense. He was not arrested or charged with wrongdoing, and just last week, St. Louis County prosecuting attorney Wesley Bell declined to charge the officer after a reinvestigation of the case.

Brown's death set off protests in Ferguson and across the country as Black citizens called for an end to police violence and racism, and led to violent confrontations with heavily armed police.

The protests elevated the "Black Lives Matter" phrase that remains central to protests across the country. Also on Sunday, protests continued in Portland, Oregon, and in Chicago, large groups of people gathered after police shot at a suspect they said had fired at officers.

In Ferguson earlier Sunday, citizens gathered where Brown was killed, with some laying flowers on the spot, CNN affiliate KMOV reported.

"Six years ago to today, this community was robbed," Ferguson Police Chief Jason P. Armstrong said to those at the site. "This nation was robbed."

Democratic Congressional candidate Cori Bush was also on hand for the gathering.

"This is where I started from, fighting for justice. We have to stand up for Black lives every single day," Bush told KMOV.

Police arrest 4 for failure to disperse

Later in the evening, officers wearing helmets and holding shields stood in lines in the parking lot facing the crowds of protesters at the police department, according to video shot by affiliate KMOV.

Police erected a barricade made of bicycle racks fastened together and put the barricade in front of the police department parking lot. They also blocked off S. Florissant Road to vehicle traffic to protect the protesters, the police said in a press release.

According to police, some members of the crowd removed the barricades, made hostile and threatening remarks to the officers and tried to enter the property.

The Ferguson Police Department received assistance from some surrounding agencies to help maintain the crowd, who police said threw bottles, metal screws and wooden sticks at officers. Some protesters ripped off letters on the building's sign and spray-painted a sign post and the road, police said.

After repeated announcements to the crowd to not throw objects or enter the parking lot, police declared an unlawful assembly and ordered the crowd to leave. Handheld pepper spray was used to disperse the crowd, police said.

Four St. Louis residents were arrested and charged with failure to disperse. One of the four charged also had an arrest warrant out of St. Louis, they said. Two officers from an assisting agency received minor injuries and were treated at the scene, according to police.