President Barack Obama has said there is "no excuse" for "excessive force" by Missouri police in a town angered by the fatal shooting of a black teenager.
The president, speaking during a working holiday in St Martha's Vineyard, Massachusetts, said officers in the St Louis suburb of Ferguson must not deny the rights of peaceful protesters.
He urged all sides to "take a step back", following Saturday's death of 18-year-old Michael Brown, which has sparked four nights of public disorder and allegations of police racism.
"Now is the time for healing," he said. "Now is the time for peace and calm on the streets of Ferguson. Now is the time for an open and transparent process to see that justice is done."
In a carefully worded statement, the Mr Obama denounced those who had attacked police, and added: "There's no excuse for police to use excessive force against protesters."
The president also said police should not be "bullying or arresting" journalists, after two reporters, one from the Washington Post and another of the Huffington Post, were detained in Ferguson.
The pair, who were taken into custody for not leaving quickly enough from a McDonald's, were later released without charge.
Police have been defending the use of tear gas and smoke bombs to disperse protesters after another night of unrest in Ferguson.
Officers clad in riot gear again faced a large crowd on Wednesday night, some of whom hurled Molotov cocktails and rocks at officers. More than 10 people were arrested.
Missouri Governor Jay Nixon meanwhile pledged that protesters would see "a different tone" from police on the streets.
Ferguson Police Chief Thomas Jackson acknowledged at a separate news conference that the community was a "powder keg", but said they would do their best to "facilitate" demonstrations.
He has said improving race relations is "the top priority right now".
The shooting has stoked racial tensions in Ferguson, where two-thirds of the 21,000 residents are black, while all but three of the police force's officers are white.
Some Ferguson residents felt that the police use of dogs for crowd control soon after Mr Brown was killed echoed a tactic from civil rights protests from half a century ago.
Police have refused to release the identity of the officer who shot Mr Brown, citing death threats and concerns for his safety.
On Thursday, the hacker group Anonymous tweeted what it said was the name of the officer.
But the St Louis County Police Department tweeted that the name leaked by the cyber-attackers was of a man who is not an officer with Ferguson police or the neighbouring St Louis County police.
Original headline: Obama Rebukes 'Excessive Force' By Police
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