Marchers protest against police shootings of two black men in Louisiana and Minnesota during a demonstration in Dallas, Texas, US July 7, 2016. [Photo/Agencies]

DALLAS - At least one sniper in Dallas killed five police officers and wounded seven more in a coordinated attack that ended when police used a robot bomb to kill a shooter who told them he wanted to kill white officers, authorities said Friday.

The attack came during one of several protests across the United States against the killing of two black men by police this week, the latest in a long string of killings that gave rise to the Black Lives Matter movement.

Police described Thursday night's ambush as carefully planned, with some shots apparently fired from rooftop positions, and said they had taken three people into custody before killing a fourth after a long standoff in a downtown garage.

"We had an exchange of gunfire with the suspect. We saw no other option but to use our bomb robot," Dallas Police Chief David Brown told reporters at City Hall.

"The suspect said he was upset about Black Lives Matter," said Brown, who is black. "He said he was upset about the recent police shootings. The suspect said he was upset at white people. The suspect stated that he wanted to kill white people, especially white officers."

In talking with police negotiators, the suspect said he was not affiliated with any group and acted alone, according to Brown.

The attack came in a week that two black men were fatally shot by police officers in Baton Rouge, Louisiana, and outside Minneapolis. The killings, both now the subject of official investigations, inflamed tensions about race and justice in the United States.

Quinyetta McMillon, who had a child with Alton Sterling, the black man slain by police in Baton Rouge, Louisiana, earlier this week, condemned the Dallas attack in a statement.

"We wholeheartedly reject the reprehensible acts of violence that were perpetrated against members of the Dallas Police Department," McMillon said. "Regardless of how angry or upset people may be, resorting to this kind of sickening violence should never happen and simply cannot be tolerated."

A Twitter account describing itself as representing the Black Lives Matter movement sent the message: "Black Lives Matter advocates dignity, justice and freedom. Not murder."

With Thursday's attack, 26 police officers have been shot and killed in the United States so far this year, according to the National Law Enforcement Officers Memorial Fund. That is up 44 percent from the 18 officers slain in the same period in 2015, the group said.

Some of the largest police forces in the United States were on high alert on Friday, following the attacks in Dallas, with departments in New York and Boston ordering officers to patrol in pairs.

PANIC IN THE STREETS

The shots rang out as a protest in Dallas was winding down, sending marchers screaming and running in panic through the city's streets.

It was the deadliest day for police in the United States since the Sept. 11, 2001, attacks on New York and Washington.

Shooters, some in elevated positions, used rifles to fire at the officers in what appeared to be a coordinated attack, Brown said.

"(They were) working together with rifles, triangulating at elevated positions in different points in the downtown area where the march ended up going," Brown told a news conference.

A video taken by a witness shows a man with a rifle crouching at ground level and shooting a person who appeared to be wearing a uniform at close range. That person then collapsed to the ground.

Reuters could not immediately confirm the authenticity of the video.

A total of 12 police officers and two civilians were shot during the attack, Dallas Mayor Mike Rawlings said. Three of the officers who were shot were women, he said.

Rawlings told CBS News the people in custody, including one woman, were "not being cooperative" with police investigators. He said the assailant who was dead was being fingerprinted and his identity checked with federal authorities.

Brown declined to say how many people were involved in the attack, saying, "We're going to keep these suspects guessing."

There was no sign of international links to the attacks, U.S. officials said on Friday.

One of the dead officers was identified as Brent Thompson, 43. He was the first officer killed in the line of duty since Dallas Area Rapid Transit formed a police department in 1989, DART said on its website. Thompson joined DART in 2009.

The department also said three of its three officers were wounded in the attack: Omar Cannon, 44, Misty McBride, 32, and Jesus Retana, 39. Retana was released from a hospital, while the other two were still being treated, a spokesman said.

US President Barack Obama delivers remarks on the police shootings in Dallas after meeting with EU leaders at the NATO Summit in Warsaw, Poland July 8, 2016. [Photo/Agencies]

'DESPICABLE ATTACK'

President Barack Obama, who was traveling in Poland, expressed his "deepest condolences" to Rawlings on behalf of the American people.

"I believe I speak for every single American when I say that we are horrified over these events and we are united with the people and police department in Dallas," he said.

Obama said the FBI was in contact with Dallas police and that the federal government would provide assistance.

"We still don't know all of the facts. What we do know is that there has been a vicious, calculated and despicable attack on law enforcement," he said.

The shooting, which erupted shortly before 9 p.m. CDT (0100 GMT), occurred near a busy area of downtown Dallas filled with restaurants, hotels and government buildings.

Mayor Rawlings advised people to stay away on Friday morning as police combed the area. Transportation was halted and federal authorities stopped commercial air traffic over the area as police helicopters hovered.

The Dallas-Fort Worth metropolitan area is one of the nation's most populous and is home to more than 7 million people.

The Dallas shooting happened as otherwise largely peaceful protests unfolded around the United States after the police shooting of Philando Castile, a 32-year-old black man, on Wednesday during a traffic stop near St. Paul, Minnesota.

The day earlier, police in Baton Rouge shot dead Sterling, 37, while responding to a call alleging he had threatened someone with a gun.

Over the last two years, there have been periodic and sometimes violent protests over the use of police force against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York. Anger has intensified when the officers were acquitted in trials or not charged at all.

Dallas is a pioneer in training its police officers in de-escalation techniques, Rawlings told reporters, saying the department had the lowest number of police-involved shootings of any large American city.

'THE END IS COMING'

The suspect in the Dallas standoff had told police "the end is coming" and that more police were going to be hurt and killed.

Police said they were questioning two occupants of a Mercedes they had pulled over after the vehicle sped off on a downtown street with a man who threw a camouflaged bag inside the back of the car. A woman was also taken into custody near the garage where the standoff was taking place.

Mayor Rawlings visited the wounded at Parkland hospital, the same hospital where President John F. Kennedy was taken after he was shot in Dallas in November 1963.

Outside the hospital, officers stood in formation and saluted as bodies of the officers were about to be transported.

Presidential candidates Republican Donald Trump and Democrat Hillary Clinton canceled planned events following the attack.

"Our nation has become too divided. Too many Americans feel like they've lost hope," Trump said in a statement. "This is a time, perhaps more than ever, for strong leadership, love and compassion. We will pull through these tragedies."

Clinton said on Twitter: "I mourn for the officers shot while doing their sacred duty to protect peaceful protesters, for their families & all who serve with them."