The one-month-old Occupy D.C. movement has grown “increasingly confrontational and violent,” the District’s police chief said, citing Friday night incidents in which some demonstrators trapped conservative activists inside the city’s convention center and four protesters were hit by a vehicle.

The police department will “adjust tactics as needed to assure safety,” Cathy L. Lanier said Monday. She did not detail those adjustments, but they could mark a turning point in the police department’s relationship with the group camped at McPherson Square.

Lanier’s remarks came as her agency is under scrutiny. On Monday, amid pressure from protesters and their attorneys, police agreed to reexamine the decision not to charge the driver in what appear to be two hit-and-run pedestrian incidents Friday at the Walter E. Washington Convention Center.

Police in other cities have moved to restrict camps and demonstrations affiliated with the Occupy Wall Street movement. District police, however, have had a largely cooperative relationship with the protesters. They have blocked off streets to allow marches without a permit, monitored McPherson Square to help keep it safe and looked the other way when instigators have taunted them.

But that could be changing, as was evident during a tense standoff Monday afternoon between officers and protesters outside police headquarters.

“The Metropolitan Police Department supports an individual’s right to assemble,” Lanier said. “We do not condone, nor will we tolerate, violence or aggression.” She described the group as peaceful last week but distributed videos Monday showing some protesters blocking the doors of the convention center and pounding on windows.

“That is no longer a peaceful protest,” she said.

Lanier’s statements seemed to reinforce comments she made in mid-October, although they were not specific to the District. “The psychology of crowds is very unique,” Lanier said at an event hosted by the Institute for Education. “I think we’re at risk for some very large, very violent protests here in the United States. And they can spark off instantly.”

Mayor Vincent C. Gray (D), who was arrested in April for blocking traffic in a voting rights demonstration, echoed Lanier’s concerns. He called on “all involved in the Occupy D.C. demonstrations to show restraint” so that the movement is not “discredited by violence.”

Leaders of Occupy D.C., who vow to continue their protest through the winter, insist they have been peaceful.

“My concern is for the safety of everyone,” said James Ploeser, 30, an Occupy D.C. organizer from Takoma Park. “We are a nonviolent, peaceful mass movement that is open, transparent. If the police chief can’t see that, she needs to watch more videos.”

And there are few signs that the National Park Service, which oversees McPherson Square, has had problems that would warrant it moving in on the group’s camp.

“We don’t have any issues with these folks in the areas of D.C. that we patrol,” said David Schlosser, a Park Police spokesman. “If there is something that needs to be addressed by us, we will address it.”

In a sign of the support that the group continues to receive, the Rev. Jesse Jackson Sr. visited the group in McPherson Square on Monday night.

Still, D.C. police have landed in the center of a widening public debate over the handling of Friday’s events at the convention center.

More than 500 protesters descended on the building to protest an Americans for Prosperity dinner honoring Ronald Reagan , blocking nearby intersections and sitting in front of some convention center doors.

At one point, according to witnesses and video, some protesters tried to storm the building. Videos posted on conservative blogs show three older attendees on the ground after they tried to leave the building. Conservative activists said the women were pushed to the ground, but protesters released their own videos Monday that they said prove they did not push anyone to the ground.

Although D.C. police had cordoned off an eight-block radius to give demonstrators unimpeded access to the streets, four protesters were injured after being struck by a vehicle.

Police stopped the suspected driver about two blocks from the scene, but no charges were filed. Instead, police issued jaywalking citations to three demonstrators who were hit. Police agreed to reopen their investigation Monday after protesters held a news conference to offer evidence suggesting that the driver deliberately struck two sets of protesters, about two blocks apart.

“One driver hit this person right here and then went around the corner and hit a family of three at the intersection,” said Adam Green. “We cannot allow it to be precedent that peaceful protesters are mowed over by cars with no proper investigation or arrest by police.”

Occupy D.C. protesters noted that the police report that referenced the driver of the car spoke of only three demonstrators struck at Seventh Street and Mount Vernon Place. The group produced a second police report taken at the same time on L Street for a “hit and run” where Georgia Pearce of Moss Point, Miss., was also struck.

Pearce, who suffered a concussion, said she was trying to cross the street near the crosswalk on Seventh Street to join another group of demonstrators when she saw a “silver four-door sedan” coming at her from “the wrong direction.”

“I was trying to get him to slow down so I could step out of the way,” Pearce said. “Instead he rushes up to me, and I end up putting my hands on the car hood, and he pushes at me and pushes me, and that is all I remember.”

Two witnesses said they saw the same car turn the corner and travel south on Seventh Street, where an Ohio couple and their 13-year-old son were struck while demonstrating in the intersection of Mount Vernon Place.

Infuriated that police did not interview them before announcing that the driver was not being charged, the victims and several dozen Occupy D.C. supporters marched on police headquarters Monday afternoon. When they arrived, a line of officers guarded the doors and refused to allow the witnesses or victims to enter the building to give statements.

After a 30-minute standoff, police agreed to allow them to enter the building to make statements.

“Hopefully, this will lead to easier actions down the road and justice for everyone,” said Heidi Sippel, who was struck at Seventh Street and Mount Vernon Place.

Staff writers Juliet Eilperin and Allison Klein contributed to this report.