US president Barack Obama has branded the shooting in Dallas, in which five police officers were killed and at least six wounded in an ambush at a protest rally, a "vicious, calculated and despicable attack".

Mr Obama, who is in Poland for a NATO summit, said he had offered federal government assistance to the city amid the "tremendous tragedy".

The Dallas drama only reached a conclusion after a two-hour standoff between one suspect, who was holed up in a building, and police, with officers sending an explosive device towards the man on a bomb robot and detonating it, killing him.

Police have revealed the suspect made a number of statements during this stand-off, including saying that he was not affiliated with any group, that he he was upset about recent police shootings of black people, and that he wanted to kill white people and white police officers in particular. He also claimed he acted "alone".

In the attack, at around 9pm Dallas time, police say five officers were fatally shot and six others were injured during a protest over recent fatal police shootings of black civilians in the US. Police originally said up to four snipers may have been involved but it was unclear in later hours whether the final suspect had in fact been the only gunman as he claimed or whether others were involved.

"As I told Mayor Rawlings," Mr Obama said, "I think I speak for every single American when I say that we are horrified over these events and that we stand united with the people and the police department in Dallas.

"We will learn more, undoubtedly, about their twisted motivations but let's be clear, there is no possible justification for these kinds of attacks or any violence against law enforcement. The FBI is already in touch with the Dallas police and anyone involved in these senseless murders will be held fully accountable. Justice will be done."

Mr Obama said it was a "wrenching reminder" of the sacrifices made by police, before hinting at the continual debate about gun control.

"We also know that when people are armed with powerful weapons, unfortunately it makes attacks like these more deadly and tragic and in the days ahead we're going to have to consider those realities as well," he said.

The protest, which had been peaceful, was drawing to an end when shots rang out near the area of Market and Main streets in downtown Dallas just before 9pm on Thursday, local time. Carlos Harris, who lives downtown, said the shooting was "strategic. It was tap tap pause. Tap tap pause."

Harris, who said he was in the military, said he heard someone fire back with an AR-15. Before the shots were fired, the demonstrators were calmly walking down Main Street. "The cops were peaceful," he said. "They were taking pictures with us and everything."

In the hours after the attack, Dallas Police Chief David Brown said it appeared four suspects had worked together to fire from elevated positions during the protest, shooting 11 Dallas police officers.

Associated Press reports a civilian, 37-year-old Shetamia Taylor, was wounded in the right calf shielding her four sons as gunfire rang out.

Mr Brown said the snipers fired upon the officers "ambush-style", with some shot in the back in a bid to kill as many as possible.

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

In the hours after the shooting, three suspects were taken into custody while one remained in negotiations with police from the garage of El Centro College in Main Street, after shooting at them for 45 minutes. He was later killed when police detonated the explosive device.

Two of the suspects were arrested after they were seen to throw a camouflaged bag into the back of a black Mercedes before speeding from the scene of the shooting. Another suspect, a woman, was arrested near to El Centro College. Their alleged role in the attack has not yet been confirmed by police.

The suspect in the garage told negotiating police that "the end is coming" and he was going to "hurt and kill more of us, meaning law enforcements", Mr Brown said. The suspect also said there were bombs "all over the place" across downtown Dallas. No explosive devices were found in a subsequent search of the area.

"We are being very careful in our tactics... as we negotiate further," Mr Brown said, in a press conference almost four hours after the attack.

"We still don't have a complete comfort level that we have all the suspects... we're likely to be working throughout the early morning hours of Friday until we're satisfied that all suspects have been captured."

Earlier, the Dallas Police Department had released an image of a "person of interest" who was wanted over the shooting, pictured wearing a camouflage t-shirt. However, it was later confirmed the man was not involved in the shooting.

Dallas Area Rapid Transit (DART) said one of its police officers was among the dead, and three other of its officers who were shot had injuries that were not life-threatening.

The protest was being held following the deaths of two men - Alton Sterling in Louisiana, and Philando Castile in Minnesota - in separate police shootings this week. The men, who were both black, were killed by police officers, and their deaths were captured on mobile phone cameras.

One witness at the protest in Dallas told Fox News that "complete pandemonium" broke out when shots were fired towards the end of the rally.

"Everyone just took off running," he said.

He believed the shooter had an assault rifle, and said police returned fire.

#breaking Crowd on the run downtown Dallas. Reports of an officer shot at the protest march. pic.twitter.com/zstZnDIRlm— Doug Dunbar (@cbs11doug) July 8, 2016

The shooting unfolded near one of the busiest parts of the city's downtown.

Footage of the panic in the moments immediately after the shooting circulated widely on social media. In many of the videos, multiple gunshots could be heard ringing out, while teams of police officers could be seen running through the area.

As the incident unfolded Clay Jenkins, a Dallas County judge and the county's chief executive said "there's no indication of who the suspects are or what their motives were, except they fired on the police."

"We are still seeking a suspect that may be held up at a downtown building, so we're asking the public to stay away. All government buildings in that area are on lockdown. That's the government centre where this is happening."

The Dallas Police Department's Twitter account had been tracking the protest's progress through the city, but abruptly stopped tweeting about 9pm, local time.

CBS news anchor Doug Dunbar tweeted that a witness reported hearing "about 20 shots in succession".

Dozens of officers were at the scene "with their guns drawn", Dunbar said.

Another witness, Michael Bautista, said he saw one police officer on the ground as he fled the scene.

"I saw the bullets hitting the cop cars," he said.

He said the protesters had been involved in a "peaceful march" before the shooting occurred.

Devante Odom, 21, told The Dallas Morning News that everyone "just started running" when the shots were fired. "We lost touch with two of our friends just trying to get out of there," Devante said.

Renee Sifflet of Dallas stood at the corner of Commerce and Houston, waiting for the chaos to die down so she could retrieve her three teenage children, who were in hiding.

"I brought them here for a positive experience, something they could say they were part of when they're older," she said. "Then it turned negative."

When they started running, she said, she lost track of her 15-year-old son for two minutes in the mayhem. "Thank God he has a cellphone," she said.

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