Dallas, Texas, emerged from a night of horror Friday that left five police officers dead, but with no certainty about what prompted the attacks and whether they have sidelined all the perpetrators.

Eleven officers - five of whom later died - and at least one civilian were injured amid a night of gun violence that started when snipers interrupted a peaceful demonstration and seemingly targeted law enforcement officials.

People had gathered in Dallas to protest police violence against African Americans across the country.

US media aired testimony from multiple witnesses stating that the violence in Dallas quickly spread across a large part of the metropolis' centre, with reports of further attacks launched by well-armed gunmen, perhaps wearing body armour.

As dawn broke, police were still reportedly in a stand-off with at least one gunman holed up in a parking garage. Media reported alternately that one gunman had killed himself and that the siege was over, but officials provided no confirmation.

Three officers were quickly pronounced dead after the first shots. Another died in one of the following attacks. A fifth succumbed to his injuries in hospital.

The attacks were "vicious, calculated and despicable," said US President Barack Obama, speaking from Warsaw, where he was due to attend a NATO summit.

"We will learn undoubtedly about their twisted motivations," he said, referring to the attackers. "But let me be clear there is no justification for these senseless attacks. ... Justice will be done."

Obama also called for prayers for the wounded before noting that the attacks once again raised the need to debate gun control in the United States.

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

At least three suspects have been arrested, Brown told a press conference early Friday. One "person of interest," an African American man wearing a camouflage shirt and holding a gun during the march, turned himself in and was subsequently released after other witnesses said he was not part of the attack.

Two suspects were pulled over after driving away from the town centre in a black Mercedes at high speed, and were being questioned on suspicion of threatening to plant a bomb, Brown was quoted as saying in the report.

But Brown said the search would continue.

"We still do not have a complete comfort level that we have all the suspects," he said.

City officials warned that large parts of the city would remain roped off Friday as much of the city centre remains an "active crime scene."

Much of the focus early Friday remained on the parking garage, near the city's El Centro College. Witnesses reported gunshots and blasts from the facility.

The suspect has said that the "end is coming" and that there are "bombs all over the place," Dallas police chief David Brown said.

One of the deceased officers was not with the city police force, but a force linked to the city's public transportation system, Dallas Area Rapid Transport (DART) reported on its Twitter site.

DART identified him as Brent Thompson, 43.

Broadcaster NBC reported that the gunshots started around 9 pm (0200 GMT Friday) in the city centre during a rally to protest two killings of black men at the hands of police officers in other parts of the country this week.

Witnesses told US media that they saw what appeared to be multiple snipers from elevated locations targeting police officers as the rally neared its end.

Authorities have said it is too early to link the attacks on the officers with the incidents being protested.

Philando Castile, 32, was shot late Wednesday near Minneapolis, Minnesota, as he tried to get out his wallet after being pulled over for a broken tail light, police said.

Alton Sterling, 37, was shot Tuesday by police in Baton Rouge, Louisiana, after officers responded to a complaint about a black male selling CDs and threatening the caller with a gun.

The US Justice Department said it was monitoring the state investigation into Castile's killing, and would conduct its own into Sterling's death.

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