A young child holds a sign as protesters demonstrate outside the Triple S Food Mart where Alton Sterling was shot dead by police in Baton Rouge, Louisiana, US, July 7, 2016. [Photo/Agencies]

DALLAS - Four police officers were fatally shot and seven wounded in one of the worst shootings of police in recent US history, by snipers who targeted them during rallies in Dallas to protest against the fatal shooting of two black men by police this week.

Dallas Police Chief David Brown told a news conference that two snipers in elevated positions shot 11 officers, killing three, in what appeared to be a coordinated attack.

A fourth officer died, police said later on Twitter. At least one more was in surgery. Some of the victims were shot in the back.

Police said one suspect whom they had engaged in a shootout had been arrested and a bomb squad unit was investigating a suspicious package found near the suspect's location.

A second "person of interest" had turned himself in, they added, though there was no word on the arrest of a possible second sniper.

The shooting happened as largely peaceful protests unfolded around the United States after the shooting of Philando Castile, 32, by police near St. Paul, Minnesota, late on Wednesday. His girlfriend posted live video on the internet of the bloody scene minutes afterward, which was widely viewed.

Castile's death occurred within a day of the shooting of Alton Sterling, 37, in Baton Rouge, Louisiana. Sterling was killed during an altercation with two white police officers. Graphic video of that incident caused an outcry on social media.