President Barack Obama called for "peace and calm" Thursday in the tense town where authorities fired tear gas at protesters and arrested reporters following the police shooting of an unarmed black teenager.
Michael Brown died Saturday in the St. Louis suburb of Ferguson, Missouri, sparking unrest and allegations of police racism.
Witnesses and law enforcement have given conflicting versions of how the 18-year-old was shot dead in broad daylight, just two days before he was due to start college.
"Now is the time for peace and calm on the streets of Ferguson," Obama told reporters. "Now is the time for an open and transparent process to see that justice is done."
With locals incensed over the incident, Wednesday marked the fourth straight night of angry protests and attempts by rifle-toting police in military-style fatigues and body armor to break them up.
Television and social media footage showed thick clouds of smoke and protesters scurrying through it. In one sequence, police fire gas at a TV news crew then dismantle its abandoned equipment.
Police also used sound bombs and rubber bullets to disperse crowds.
Noting the raw emotions, Obama said local authorities had a responsibility to be "open and transparent" about how they were investigating Brown's death and protecting the community.
"There is never an excuse for violence against police or for those who would use this tragedy as a cover for vandalism or looting," he said.
"There's also no excuse for police to use excessive force against peaceful protests or to throw protesters in jail for lawfully exercising their first amendment rights."
Amid Wednesday night's tension, police also arrested, and later released without charge, two journalists covering the events.
Wesley Lowery, a Washington Post political reporter, and Ryan Reilly, who works for the Huffington Post, were detained in a McDonald's after police entered the eatery and ordered people to leave, the pair wrote on Twitter.
Obama referenced the incident, warning: "Here in the United States of America, police should not be bullying or arresting journalists who were just trying to do their jobs and report to the American people on what they see on the ground."
The US leader, vacationing on the Massachusetts island of Martha's Vineyard, said he spoke with Missouri Governor Jay Nixon, who was due to make a statement amid reports he would be removing the St. Louis County police from Ferguson amid criticism of their conduct.
Earlier, Nixon called the situation in the town deeply troubling, adding it "does not represent who we are as Missourians or as Americans."
"While we all respect the solemn responsibility of our law enforcement officers to protect the public, we must also safeguard the rights of Missourians to peaceably assemble and the rights of the press to report on matters of public concern," he said in a statement.
- 'We have to get answers' -
As day broke, the Brown family's attorney had also appealed for calm and urged federal authorities to carry out an independent autopsy of their son's body.
"There is nothing Michael Brown could have done to justify him being executed in broad daylight, and this is the worst police shooting I have ever seen," Benjamin Crump told CNN.
"We have to get answers and it has to be transparent and that's why we're calling on the Justice Department and the attorney general to do an independent autopsy."
Obama, in his remarks, said he had already tasked the FBI to independently probe Brown's death along with local officials on the ground.
He added that he had made clear to Attorney General Eric Holder "that we should do what is necessary to determine exactly what happened and to see that justice is done."
- Media arrests 'unacceptable' -
The arrests of the journalists also sparked international reaction, with the Vienna-based Organization for Security and Cooperation in Europe -- better known for reporting on rights abuses and poor governance in conflict zones -- calling the incident "unacceptable and a clear violation of the right of media to cover news."
"Summarily rounding up journalists while they are doing their jobs sends a dangerous precedent and must never be condoned," the group's media freedom watchdog Dunja Mijatovic said.