Protests against the police killing of two African-American men, Alton Sterling and Philando Castile, in two separate incidents turned tense in Phoenix Friday.

Thousands of protesters gathered across the country to express their anger against the brutal killing of Sterling and Castile. The protests come just a day after Micah Johnson gunned down five police officers in Dallas.

In Phoenix, the situation became tense when officers tried to prevent the protesters from blocking an important highway. The protest reportedly began at 8 p.m. local time with about 500 protesters marching through the streets of Phoenix.

Police in riot gear used pepper spray on protesters and arrested three, the Phoenix Police Department reportedly said. Six people were also injured. “Protesters throwing rocks at Phoenix PD,” spokesman for the Arizona State Troopers Damon Cecil tweeted.

According to CNN, the police had formed a human shield to prevent protesters from blocking the highway. Protesters threw rocks at the police after officers used pepper spray on them.

The Arizona Department of Transportation said that it had closed parts of Interstate Highway 10 due to the protest organized by Baptist preacher and civil rights activist Rev. Jarrett Maupin.

Atlanta reportedly saw the largest demonstration with nearly 2,000 people blocking an interstate ramp during a march organized by the National Association for the Advancement of Colored People (NAACP). Demonstrators first gathered at the National Center for Civil and Human Rights near Centennial Olympic Park and later moved to the Atlanta Connector, where two major highways intersect.

Atlanta Mayor Kasim Reed also attended the march, CNN reported, and said that peaceful protesters were practicing their First Amendment right. But he criticized protesters’ decision to block the highway. He tweeted, “If you enter the highway, you endanger your own life, the lives of innocent motorists & the lives of our officers. We are better than that.”

“What's not really happening to my great disappointment in the United States of America right now is after these incidents are videotaped and in many instances after the judicial process has completed, there seems to be a cheapening of the life of black people in that folks are walking away from incidents that I am stunned that they've walked away from,” Reed reportedly said. He later tweeted saying that ten protesters were arrested.

Similar protests reportedly broke out in Arkansas, California, Illinois, Michigan, Nebraska, New York, Pennsylvania, Utah, Washington D.C., Massachusetts, Louisiana and Colorado.

President Barack Obama is cutting his Europe trip short to visit Dallas next week. The president, who had a tour of Spain scheduled, will instead visit the site of the deadly shooting in an effort to “bring people together to support our police officers and communities, and find common ground by discussing policy ideas for addressing the persistent racial disparities in our criminal justice system,” his press secretary Josh Earnest reportedly said.