07:32 GMT - We are closing our live report on the shooting dead of five police officers and wounding of six other officers and a civilian by snipers during a protest march in Dallas.
Three people are in custody, two men and a woman, while police are continuing in a standoff with another suspect holed up in a downtown garage.
Police Chief David Brown has warned that "we still don't have a complete comfort level that we have all the suspects."
Several hundred people attended the Dallas rally, one of several nationwide over the fatal police shootings of black men in Minnesota and Louisiana this week.
In a poignant moment, Dallas Mayor Mike Rawlings spoke of a "heartbreaking morning" for the city.
Sports world reacts
07:10 GMT - The sports world is reacting with shock to the shootings:
"Hurting for Dallas right now," says Alex Morgan, a member of the US national women's soccer team.
"It seems like we wake up or go to sleep reading more tragedies than not. I want to wake up from this nightmare."
NBA star LeBron James takes to Twitter to tell his 31.5 million social media followers violence won't solve anything.
"We are all hurting tonight," James tweets. "More violence is not the answer. #StoptheViolence."
"I want to hope this is all a bad dream...it ain't," two-time Major League Baseball World Series champion Shane Victorino tweets.
Chris Iannetta, catcher for the Seattle Mariners, calls for an end to shootings on both sides.
"People. Please end the violence. Enough is enough. We are all humans. We all deserve the right to live happily!"
Japanese pitcher Yu Darvish has played four seasons with the Texas Rangers and lives in Dallas.
"Pray for my hometown. #prayforDallas," he tweets.
"Praying for this world my kids gotta grow up in...I'll die for them, won't let the streets or the (social) media raise them. That's on God," tweets Oklahoma City Thunder player Anthony Morrow.
Spencer Dinwiddie, who played 12 games with the Detroit Pistons this past season, said the killings seem so senseless.
"Whole lotta hate out here...and in the grand scheme of things it's for no reason," he tweeted.
07:16 GMT - The White House says President Barack Obama, who is attending a NATO summit in Warsaw, has been briefed.
"The president has been updated on the shooting of police officers in Dallas. He asked his team to keep him updated on the situation as they get additional information," White House spokesman Josh Earnest says.
NHL team Dallas Stars prays
06:38 GMT - Here is a summary of events so far.
Four police officers have been shot and killed and seven others wounded by snipers during a protest march in Dallas against police shootings of black men.
A civilian has also been wounded.
Police are continuing to exchange gunfire with a suspect holed up in a downtown garage hours after the shooting started.
The suspect "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," Police Chief David Brown has told reporters.
A woman who had been in the same part of the garage is in custody, along with two suspects who had camouflage bags in a Mercedes and a person of interest who turned himself in after police tweeted a picture of him wearing camouflage and an assault rifle slung across his shoulder.
Brown has warned that "we still don't have a complete comfort level that we have all the suspects."
Several hundred people attended the Dallas rally, which ended just before the shots rang out around 9:00 pm (0200 GMT Friday).
The protest was one of several nationwide over the fatal police shootings of black men in Minnesota and Louisiana this week that have prompted US President Barack Obama to make an emotional appeal for urgent police reform.
Dallas Mayor Mike Rawlings said both the White House and Texas Governor Greg Abbott had offered to help.
In a poignant moment, Rawlings spoke of a "heartbreaking morning" for the city.
Mavericks owner Mark Cuban
06:09 GMT - Mark Cuban, owner of the NBA's Dallas Mavericks basketball team, calls on people of Dallas to unite:
06:11 GMT - The US Federal Aviation Administration is restricting air movements over Dallas to help police operations.
The restrictions, which are due to last from 0335 to 1130 GMT cover a radius of 2.5 nautical miles.
"Only relief aircraft operations under direction of Dallas Police Department are authorized in the airspace."
Courage, professionalism, grit
05:58 GMT - Brown says "one of our officers who was in surgery that we had hopes would survive passed, bringing the total count of officers that passed away to four. Three Dallas police officers and one DART (Dallas Area Rapid Transit) officer.
"We're continuing to ask for the public's prayers and support and encouragement, but I could just tell you I've never been more proud of being a police officer and being a part of this great noble profession, seeing the courage, the professionalism, and just the grit to stay on scene in an area looking for suspects, knowing we are vulnerable."
"...some of the stories I've heard from officers talking about what happened, running toward gunfire to help the injured officers, to get them transported to the hospital by patrol car. Not having the time to wait for an ambulance and just so many stories of great courage. So we continue to fight the good fight to bring this to a conclusion."
05:55 GMT - "We still don't have a complete comfort level that we have all the suspects. So we will continue a very, very rigorous investigation and search of downtown.
"We'll likely be working throughout the early morning hours of Friday. Until we are satisfied that all suspects have been captured and have an opportunity to be interviewed so that we can fully understand what has motivated this attack on our officers.," says Brown.
'Bombs all over the place'
05:53 GMT - The suspect "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," Brown tells reporters.
"So we are being very careful in our tactics so we don't injure our citizens in Dallas as we negotiate further."
05:50 GMT - Brown said police are currently negotiating with a suspect "involved in these shootings at the garage of El Centro in downtown Dallas.
"This suspect we're negogiating with for the last 45 minutes has been exchanging gunfire with us and not being cooperative in the negogiations. Before I came here I asked for plans to end this standoff and as soon as I'm done here. I'll be presented with those plans."
Kill more officers
05:44 GMT - Brown says the suspects have said they have placed bombs in the area to kill more police officers.
05:38 GMT - Mayor Rawlings is appealing for people to stay away from the crime area.
"It's a heartbreaking morning to lose these four officers who proudly serve our citizens."
05:37 GMT - Brown praises the courage, professionalism and grit of his officers.
Search goes on
05:35 GMT - Brown says police are still unsure if they have all the suspects and would continue to search "until we are satisfied all suspects have been captured."
05:33 GMT - Dallas Mayor Mike Rawlings is holding a press conference with police chief David Brown.
'Lose our chains'
05:24 GMT - In the other protests around the US dozens of mostly young people blocked traffic on a highway in Chicago, chanting: "It is our duty to fight for our freedom. It is our duty to win. We must love and support each other. We have nothing to lose but our chains."
About a thousand protesters clogged New York's iconic Times Square, at one point staging a sit-in. Police arrested some of them, loading them onto city buses they borrowed for the occasion.
Another thousand rallied in Washington at the White House, then marched to the US Capitol where veteran civil rights leader Congressman John Lewis stood on the steps and addressed the crowd through a bullhorn.
05:27 GMT - US aviation authorities have now restricted flights over Dallas following the deadly shooting.
'An American issue'
05:13 GMT - On arrival in Poland for a NATO summit President Obama went further.
"When people say black lives matter, it doesn't mean blue lives don't matter," referring to the color of uniforms worn by many American police officers.
"It's incumbent on all of us to say we can do better than this."
"This is not just a black issue. It's not just a Hispanic issue," he said. "This is an American issue that we should all care about."
05:03 GMT - The protest in Dallas was part of a series of demonstration in several cities over the deaths this week of Alton Sterling in Louisiana and Philando Castile in Minnesota -- both caught on video.
Earlier President Barack Obama made an emotional appeal for urgent police reform.
"They are symptomatic of the broader challenges within our criminal justice system, the racial disparities that appear across the system year after year, and the resulting lack of trust that exists between law enforcement and too many of the communities they serve," he said in a statement on Facebook.
Obama urged Americans to work together to heal the divisions, while making clear that police officers should not be blamed wholesale.
"To admit we've got a serious problem in no way contradicts our respect and appreciation for the vast majority of police officers who put their lives on the line to protect us every single day," he said.
Thousands have taken part in the demonstrations marching and chanting across Los Angeles, Chicago, Washington, Dallas and Atlanta, with more than 1,000 protesters gathering in New York's Time Square.
'High rate of speed'
04:59 GMT - Dallas police say an officer spotted an individual on Lamar Street in downtown Dallas who threw a camouflaged bag into the back of a Mercedes which drove off at a "high rate of speed".
Officers followed the vehicle southbound on Interstate-35 and performed a traffic stop and are now questioning both occupants of the vehicle.