Five Dallas police officers were shot dead Thursday night in Dallas, in what President Barack Obama described as a "vicious, calculated, and despicable attack."

In addition to the five slain law enforcement officials, seven other officers and two civilians were wounded and one gunman is dead after an overnight standoff.

On Friday afternoon, thousands of people attended an interfaith service at a park in downtown Dallas to mourn the victims and condemn the violence. Across social media and in public outpourings, people from across the nation and around the world lamented the loss of life.

The Thursday night attack, described widely as an ambush, took place as an anti-brutality march and rally, one of many across the U.S. protesting this week's fatal police shootings of Alton Sterling in Louisiana and Philando Castile in Minnesota, came to a close.

At a press conference Friday morning, Dallas Police Chief David Brown said the dead gunman was killed by a bomb detonated by authorities after negotiations broke down. Brown said that before he died, the suspect told police he was not affiliated with any groups and was "upset about Black Lives Matter...upset about the recent police shootings...upset at white people."

Earlier, Brown said two shooters used sniper rifles to fire at the officers. "(They were) working together with rifles, triangulating at elevated positions in different points in the downtown area where the march ended up going," he reportedly said. 

According to news reports, authorities have three people in custody—a woman and two people who were in a car stopped on a road. 

The shootings come as the nation mourns Sterling, a black man killed in Baton Rouge on Tuesday after being pinned to the ground by two white officers, and Castile, who was shot by a police officer on Wednesday in a Twin City suburb in front of his girlfriend and her four-year-old daughter.

The Dallas Morning News reported that the Dallas rally had been organized just 24 hours earlier by Dr. Jeff Hood, a preacher, and Dominique Alexander, the founder of the Next Generation Action Network. "Hood had hoped a rally led by a white preacher and the black Next Generation leader would show unity and relieve anger," the Morning News writes.

The two organizers  are expected to speak at a news conference at 11 am Friday outside Dallas City Hall.

"We want Dallas to know that violence of any kind we condemn," Alexander told the Morning News in a 3 am phone interview. "We continue to stand with the families of these officers and pray with them, as well as we stand with the families of Alton Sterling and Philando in Minnesota."

Added Hood: "We didn't want anyone else to die. That is the reason we did the protest. ... Violence doesn't heal anybody."