CNN and other media reported that police officers in riot gear marched toward the protesters near a burned out gas station yesterday where demonstrators had gathered in the town of Ferguson, a St Louis suburb.
Police separately arrested two journalists reporting on the unrest that has roiled Ferguson since aspiring college student Michael Brown, 18, died Saturday in a police shooting.
His death has triggered rioting and stirred comparisons to the February 2012 fatal shooting of unarmed teenager Trayvon Martin in Florida.
In a series of tweets, Lowery said they were given no explanation for their arrest other than "trespassing" and were not charged with any offenses before being released.
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"I'm emotional, but need to note: Ryan and I are fine. Have seen people in Ferguson hurt by gas/rubber bullets. This wasn't that," Lowery tweeted.
Lowery also said the police officers "assaulted" him because the two reporters were not leaving the McDonald's quickly enough.
"SWAT just invade McDonald's where I'm working/recharging. Asked for ID when I took photo," he wrote.
Earlier, police fired several shots at a 19-year-old who pointed a handgun in their direction as they dispersed around 30 people who had gathered near where the rioting had occurred, according to the St Louis Post-Dispatch.
The young man was taken to hospital and his name has not been released.
In Los Angeles, police shot and killed a black man on Monday during what they called an "investigative stop."
"It is unknown if the suspect has any gang affiliations," the Los Angeles Police Department said in a statement.
Officials in Ferguson urged demonstrators to remain calm.