COEUR D’ALENE, Idaho — Dozens of members of a white supremacist group were arrested on Saturday in Idaho before they could act on plans to riot at a local Pride event, the police said.

After receiving a tip from a concerned citizen, the police detained and charged 31 people who belonged to a far-right group known as Patriot Front, said Lee White, the chief of the Coeur d’Alene Police Department, at a news conference.

They are being charged with conspiracy to riot, a misdemeanor, he said.

Bob Norris, the sheriff of Kootenai County, said that a person reported seeing a group of people jump into a U-Haul van near the intersection of Northwest Boulevard and Interstate 90 in Coeur d’Alene.

“And they were all dressed like a small army,” Sheriff Norris said. “We had units in their area, and we were able to intercept them pretty quickly.”

A few miles away, the North Idaho Pride Alliance was holding “Pride in the Park,” an annual event, at Coeur d’Alene City Park.

The Anti-Defamation League, which tracks extremist organizations and hate crimes, describes Patriot Front as a Texas-based white supremacist group that formed when members of another white supremacist group, Vanguard America, broke off after the Unite the Right rally in Charlottesville, Va., in 2017.

The members arrested had come to Idaho from several states, the police said, including Texas, Utah, Colorado, South Dakota, Illinois, Wyoming, Washington, Oregon and Virginia. At least one of the members lived in Idaho, the authorities said.

According to the A.D.L., Patriot Front frequently participates in “flash demonstrations,” which are designed to create viral video content and for which members generally wear masks and “khaki pants and a blue or white polo shirt,” and sometimes employ smoke bombs.

Chief White said those arrested were wearing khaki pants as well as arm patches and hats emblazoned with Patriot Front logos. Videos of the arrest circulating on social media show men on their knees with their hands tied behind them. Many of the men are masked and wearing blue shirts.

“If you go online, look up ‘Patriot Front,’ that’s exactly how these individuals are dressed,” Chief White said.

Many of the men also had shields and wore shinguards, and the police recovered one smoke grenade, they said. They did not mention other weapons.

“I have no doubt in my mind, they were coming downtown to riot,” Chief White said.

Chief White denied online rumors claiming that the arrests had stemmed from the work of informants.

“This all came from a concerned citizen,” he said.

An apparent leader of the group had a seven-page document outlining an operational plan in extensive detail, the chief said.

After pulling up a digital image of the document, Chief White read brief selections to The New York Times that detailed how smoke was to be used: “a column forming on the outside of the park, proceeding inward, until barriers to approach are met” and “once an appropriate amount of confrontational dynamic has been established the column will disengage and head to Sherman.”

Sherman Avenue runs through the center of downtown Coeur d’Alene.

Kootenai County jail records revealed that Thomas Rousseau, the founder of Patriot Front, was among those booked on criminal conspiracy charges.

In the weeks leading up to the Pride event, Sheriff Norris said, “there was a lot of chatter going on” from both far-right and far-left sources about potential confrontations at the gathering.

Some of that came from local groups, including the Panhandle Patriots, a far-right motorcycle club in northern Idaho. But several of those groups publicly altered their plans as worries of a violent confrontation increased. The Panhandle Patriots rebranded a planned “Gun d’Alene” anniversary event as a “North Idaho Day of Prayer.”

Chief White said that, at this point, he did not have information suggesting that local members of the alt-right or of other far-right groups were connected to Patriot Front’s plans.

Chief White said there were members of antifa groups present at the Pride event. But the Pride in the Park attendees “felt relatively safe, at least the event organizer did,” Chief White said.

“There were people walking around the event with long guns and handguns and bear spray and all kinds of things like that,” he said. But, he added, “that is legal in Idaho.”

Sheriff Norris characterized the arrests as a victory for the community and for Kootenai County law enforcement. Without the actions of the police and the call from the individual, he said, “we would be talking about a different situation.”