RICHMOND — City police arrested 23 people over the weekend after demonstrations for social justice turned violent on Saturday and Sunday nights, interrupting more than three weeks of largely peaceful protests.

Officials blamed outsiders for an escalation that saw about $100,000 in damage to windows at Virginia Commonwealth University, a city dump truck set ablaze and one man caught on video firing a gun into the ground in a confrontation with protesters.

“There were white supremacists marching under the banner of Black Lives Matter, attempting to undermine an otherwise overwhelmingly peaceful movement towards social justice,” Mayor Levar Stoney said Sunday at a news conference.

He offered little direct evidence of outside influence, though officials said Monday that was still under investigation. Police Chief Gerald M. Smith said that fliers promoting anti-police rallies for the weekend had been produced outside of Richmond and that several of the people arrested were from other parts of the state.

Demonstrations have been going on virtually every night in Richmond since May 31, triggered by the killing of George Floyd in Minneapolis police custody a few days before. The first night or two featured looting and broken windows, and police have used chemical agents several times to disperse crowds.

But for much of the past month, as city and state leaders began pulling down the city’s Confederate statues and pledged to address racial inequities, the protests have had the air of civic-minded street festivals, with music, speeches and public art.

Late last week, the anti-police fliers began circulating on social media, calling for rallies on Saturday in sympathy with protesters in Portland, Ore., where unbadged federal troops have been using tear gas and harsh tactics against demonstrators.

That night, several hundred protesters staged a confrontation outside the heavily fortified headquarters of the Richmond police. The cab of a city dump truck was set ablaze, along with two dumpsters.

Police later released a photo of batteries, rocks and bricks that they said protesters had thrown at officers that night. Smith said firefighters responding to the burning truck were also pelted with bricks.

Later that night, video posted to social media showed a white man jump out of a pickup truck and confront a small number of protesters while appearing to fire a handgun into the ground. No one was injured. Smith said Monday that the incident was under investigation and that no arrests had been made.

Six people were arrested Saturday night — five charged with unlawful assembly and the sixth with assault of a police officer.

On Sunday night, after a similar flier was distributed, police “took a proactive approach,” Smith said Monday. Officers were waiting at Monroe Park, a gathering place for protesters that closed at 10 p.m. When people failed to disperse after closing, police moved in and made 17 arrests — including at least one reporter for the VCU campus newspaper who was covering the protests.

The charges included trespassing, rioting, possessing a weapon with an extended magazine and possession of cocaine with intent to distribute.

Officials supplied little direct evidence to back up claims that the weekend’s violence was perpetrated by outsiders, though Smith said that was still under investigation. Nine of the 15 adults arrested were from Richmond or nearby Henrico County, and the rest were all from Virginia.

Stoney displayed a photo of a plywood sign painted to say “Black Lives Matter” that he said a white supremacist had carried as a shield.

“The mission is simple,” Stoney said Sunday. “To undermine the months of peaceful, community-driven protests that this city has seen.”