Thousands protest against police violence in the US.

Reuters Thousands of people took to the streets in several cities across the United States on Friday to protest the shooting deaths of two black men by police officers earlier in the week.

The protests occurred a day after a gunman killed five police officers who were monitoring similar demonstrations in Dallas, Texas.

Protesters filled the streets of New York, Atlanta, and Philadelphia. Events in San Francisco and Phoenix also drew large crowds.

Six people were injured and three were arrested in Phoenix, where riot police used pepper spray against protesters, some of whom were throwing rocks at officers, according to a statement from the Phoenix Police Department.

Videos posted online showed protesters also gathering in Baton Rouge, Louisiana, where 37-year-old Alton Sterling was shot and killed by police on Tuesday morning.

The largest demonstration appears to have taken place in Atlanta, where thousands marched, chanting and demanding justice, according to videos posted on social media. Atlanta Mayor Kasim Reed tweeted that the protest was largely peaceful, although 10 people were arrested.

Friday marked the second day of protests against police use of force, following the shooting deaths of Sterling and 32-year-old Philando Castile outside St. Paul, Minnesota.

Castile was killed by police during a traffic stop on Wednesday. Minutes later, his girlfriend posted footage of the incident online.

Thursday's demonstrations had been mostly peaceful until shots were fired at a dispersing protest in Dallas.

Authorities said Micah Johnson, a 25-year-old Afghanistan veteran who stated he wanted to "kill white people," launched a long-range shooting attack that killed five police officers and wounded nine others.

The gunman was killed by police using a robot loaded with explosives after he was cornered in a parking lot.