It is the deadliest single attack on U.S. law enforcement since the 2001 terror attacks.

Five Dallas police officers have been shot dead while seven others are injured and undergoing surgery. Two civilians have also been injured in the attack. Three suspects are in custody, another died after a stand off with police at a downtown parking garage.

It happened when gunfire rang out as demonstrators in Dallas marched against the shooting deaths of two African American men by police in Louisiana and Minnesota.

Protesters expected the protest to be a peaceful event.

“Credit needs to be given to the Dallas Police Department and the effort and job that they did, saving so many lives and really standing up and fighting to get their own who were injured to hospital as quickly as possible to save their lives as well,” said photojournalist Josh Steven who was covering the event.

U.S. President Barack Obama has offered his comments at a news conference in Warsaw, Poland where he is attending the NATO summit.

“Today, our focus is on the victims and their families,” Obama explained, “They are heartbroken, the entire city of Dallas is grieving. Police across America, a tight knit family, feels this loss to the core, and we’re grieving with them.”

Obama says the FBI has already been in touch with Dallas police, and is promising to hold those responsible to account.