A Philippine police van rammed and ran over baton-wielding protesters outside the US embassy in the capital, Manila.
Police also used tear gas and truncheons on Wednesday to try to disperse about 1,000 people who had gathered in support of President Rodrigo Duterte's recent comments that he wanted to loosen his nation's alliance with the United States.
Police made at least 23 arrests, said Chief Inspector Arsenio Riparip, one of the officers overseeing the incident, adding demonstrators broke through the line of police securing the embassy's gates.
The van's driver, police officer Franklin Kho, told reporters he was driving the van away from the protesters because he feared they would try to seize it and use it to attack the police.
Photos showed a grey-haired man trapped underneath the stationary van with his leg and hips under one of the back tyres.
The rally came as Duterte visits Beijing to strengthen relations with the world's second-largest economy amid deteriorating ties with former colonial power, the US.
"We had to disperse them. They started it. They were trying to enter the embassy," Riparip told AFP news agency.
"We had to use tear gas. They overpowered our policemen."
A police van reversed quickly back into a crowd of dozens of protesters and then forward, running over at least two people and banging into others, footage broadcast by local television network ABS CBN showed.
Some screamed in surprise, others hurled stones at the van and yelled invectives. A speaker called the police "puppies of imperialists" on a loudspeaker.
A fire truck doused the rowdy protesters with water to push them back, but they took hold of the water hose and confronted the outnumbered police with rocks and red paint. After breaking through the police corridor, they scribbled "US troops out now" and other slogans on the embassy's tall fence with red paint.
Manila police chief Oscar Albayalde confirmed that the van hit protesters but insisted the driver was not at fault.
"They weren't really run over," Albayalde said in a statement, referring to the protesters.
"The rallyists were trying to flip over the patrol car. In the process, the driver extricated the patrol car and inadvertently hit some unruly protesters who sustained minor injuries."
The protesters were calling for the removal of US troops in the southern island of Mindanao.
"There was absolutely no justification [for the police violence]," Renato Reyes, secretary general of left-wing activist group Bayan, said. "Even as the president avowed an independent foreign policy, Philippine police forces still act as running dogs of the US."
One of the protest leaders, Amirah Lidasan, accused the police of starting the violence.
"It was the police who attacked the protesters. First they rammed the police vehicle against the people. Then they released tear gas and hit us with truncheons," she said.
While the Philippines is a defence ally of the US, Duterte - elected president in May on a ruthless anti-crime platform - has said he wants to distance the country from the United States.
He has launched abusive tirades against US President Barack Obama and warned that he may eventually cut ties altogether, after the US government raised alarm over thousands of people killed in Duterte's anti-drug campaign.