The attack in Dallas will strain an already tight Dallas police force, writes the Dallas Morning News:

"Since October, 195 officers have left the Dallas Police Department. Many of those have retired, but more than 50 officers left to work at other police agencies. 

"In recent months, many officers have been working overtime and non-patrol officers have filled patrol shifts to respond to calls.

Image source, AP

The parking garage in which police are locked in a standoff with the final suspect is next to El Centro College on 801 Main St. The college campus was placed on lockdown ahead of the protest, as a precaution - and remains in that state.

Several reports on Twitter say a series of explosions have been heard from the vicinity of the parking garage where the final suspect is locked in a standoff with police.

One Dallas reporter said he heard a "loud boom" after being told to move by police.

Some reports say the explosions are part of a police operation to end the standoff.

Video footage has emerged showing the moment protesters first ran for cover as shots were fired at police.

Dallas mayor Mike Rawlings and police chief David Brown have arrived at Parkland Hospital in Dallas - where John F Kennedy died - to visit the injured. 

US aviation authorities are restricting flights over Dallas.

A Federal Aviation Authority notice to pilots reads: "No pilots may operate an aircraft in the areas covered by this NOTAM.

"Only relief aircraft operations under direction of Dallas Police Department are authorized in the airspace."

Police have three suspects in custody and are in a standoff with a fourth.

• A suspect is holed up in a parking garage and has said he intends to kill more police and detonate bombs
• Two men were arrested after a police officer saw a man throwing a camouflage bag into a Mercedes near the scene and speeding off. 
• Another female suspect was arrested near the scene

• Police chief David Brown said the suspects in custody are not co-operating

Four officers have died. 

• Three Dallas police officers and one Dart transit officer were killed
• Seven other officers were shot and injured, some critically
• One civilian was wounded

Speaking at the news conference, Dallas Mayor Mike Rawlings said: "It is a heart-breaking morning to lose these officers that proud served our city."

He said the White House had offered assistance to the city, and that he and Mr Brown were going to the hospital to visit the injured.

Mr Brown said there were four suspects, who were "working together with rifles, triangulating from elevated positions".

He said two suspects already in custody were not co-operating with police to give information about their motives.

"We are being very careful in our tactics so we don't put anyone in harms way," Mr Brown said.

"We still don't have a complete comfort level that we have all the suspects in custody."  

Police are still negotiating with one suspect who is holed up in a parking garage. He has threatened to kill more officers and detonate bombs, Dallas Police chief David Brown said. 

He said: "The suspect that we are negotiating with that has exchanged gunfire with us over the last 45 minutes has told our negotiators that the end is coming, and he is going to hurt and kill more of us, meaning law enforcement, and that there are bombs all over the place in this garage and in downtown.

"We are being very careful in our tactics so we don't injure anyone or put anyone in harm's way.

"We still don’t have a complete comfort level that we have all the suspects, so we will continue a very very rigorous investigation … until we are satisfied that all suspects have been captured."

Several pieces of video footage, both amateur and from reporters, captured the sound of gunfire ringing out in Dallas.

The Dallas Morning News has published its front page.

The Dallas Police Association has issued a statement saying it is "horrified" by the events.

Dallas Police Department says it has arrested two men after one was seen near the incident throwing a camouflage bag into a car and driving off. Here's their full statement:

Image source, DPD

The Mayor of Dallas, Mike Rawlings, has said it is a "heart-breaking moment for the city".  

One of the protest march's organisers, Reverend Jeff Hood, said he watched people scramble for cover as gunfire rang out.

"I ran away from the shots trying to get people off the streets and I was grabbing myself to see if I was shot," he told the Dallas Morning News.

Dallas Police Chief David Brown initially said 11 officers were shot "ambush style" by snipers.

"We believe that these suspects were positioning themselves in a way to triangulate on these officers from two different perches... and planned to injure and kill as many law enforcement officers as they could," he said.