Right-wing marchers and hundreds of counterprotesters turned the grounds of the state Capitol into a bloody melee Sunday, injuring 10 people — including two in critical condition with stab wounds — in a violent clash that erupted despite a heavy law enforcement presence, CHP and fire officials said.

The fight broke out between members of a group calling itself the Traditionalist Worker Party and the counterprotesters, some of whom were members of the Black Lives Matter movement, said Chris Harvey, the Sacramento Fire Department’s public information officer.

“My understanding is we had Black Lives Matter, KKK and some other right-wing groups,” he said. “There was a wide range of individuals here today.”

It all started on the south steps of the Capitol with smaller skirmishes spreading throughout the entire grounds — an area six blocks long and three blocks wide, added Harvey.

At around 9 a.m., the counterprotestors began amassing.

Their numbers swelled to 400-plus, said George Granada, a California Highway Patrol public information officer with the Capitol Protection Division.

“They were staged, basically, around the Capitol, out on the city street. On the south side, west side, north side of the Capitol looking to be: ‘Hey, where are these guys going to try to enter from?’” Granada said.

At 11:45 a.m., roughly 30 Traditionalist Worker Party members arrived.

“And then, that word spread quickly through the antiprotester (group) from whatever location that they were at, they started running to that south side of the Capitol,” Granada said. “I don’t think there was any kind of verbal anything. I think it was an immediate brawl that took place.”

No arrests were made, and at least 100 law officers were present during the clash, Granada added. The Fire Department treated nine men and one woman, ages 19 to 58, for injuries that included cuts, scrapes, bruises and lacerations, Harvey said. Nine were taken to area hospitals, while one person declined further treatment.

The Traditionalist Worker Party is a white nationalist, political group that formed early last year, according to the Southern Poverty Law Center.

Headquartered in Bloomington, Ind., the group has roughly 500 members across a dozen chapters in about half a dozen states, said Matt Parrott, the group’s press coordinator, who works as a truck driver.

The Traditionalist Worker Party apparently came expecting a fight, previewing the day’s events as its own version of the movie “300,” according to a Web page advertising a live stream of the march.

“After carefully weighing the pros and cons, we have decided that this would be our Thermopylae,” wrote Matthew Heimbach, the chairman of the Traditionalist Worker Party, referencing the ancient Greek battle that inspired the Hollywood film. “No matter what it costs or what it takes the march will go on!”

Of the 30 men who showed up, at least one was “stabbed in the arm, and it ended up nicking up one of his arteries,” Heimbach, a landscaper, told The Chronicle by phone. “They had to use a belt to slow down the bleeding, but he lost a lot of blood before he had to go to the hospital.”

He said the person is expected to recover.

Heimbach cut the call short, saying he was receiving a “call from a comrade in Moscow.”

Neither Parrott nor Heimbach was at the rally, Parrott added. Both live in Indiana.

Parrott said the march was organized roughly two months ago and promoted over Facebook and Twitter.

Meanwhile, a group called Anti-Fascist Action Sacramento, which also calls itself Antifa Sacramento, planned the counterdemonstration.

In a “call to action” first posted May 19, Antifa Sacramento asked counterdemonstrators to show up on the Capitol’s west steps by 9 a.m. Sunday to “confront those bigots, deny them a platform to promote hate and to make sure they know they are not welcome on our streets or in our communities,” according to a flyer posted on the group’s Web page and promoted on Facebook.

The group said Nazis, skinheads and the Ku Klux Klan were planning to demonstrate at the Capitol.

“Anti-Fascist Action Sacramento does not believe in allowing hate to have a platform, and we are calling upon the community to shut down their rally,” the group said. “Fighting fascism is a moral duty, not a political one.”

Ben Briskin, an Antifa Sacramento organizer, said about 300 counterdemonstrators came from as far as Oregon and Washington.

Briskin said the melee started when several of the Traditionalist Worker Party members tried to sneak into the Capitol building through a different entrance than originally planned.

“A bunch of us rushed over to try and block their path,” Briskin said in a telephone interview. “Some of them split off from the main group and started attacking the people we had.”

Briskin said he saw two counterprotesters, both men, stabbed in the stomach or below the rib cage, and added that his organization counted a total of five who were stabbed.

At that point, “all hell broke loose,” Briskin said. The counterprotesters began fighting back, eventually chasing the demonstrators away from the Capitol, he said.

“We went after them with fists, sticks, whatever we had, anything that would get them out of there,” he said.

Briskin said his group, which represents a “wide range of antifascist and antiracist organizations,” anticipated violence at the rally based on the worker party’s history and “other fascist rallies in general.” More than two weeks ago, Antifa Sacramento started a crowdfunding campaign on Rally.org to raise money for bail and medical expenses.

Sean Sposito and Benny Evangelista are San Francisco Chronicle staff writers. Email: ssposito@sfchronicle.com, bevangelista@sfchronicle.com Twitter:@ChronicleBenny, @SeanSposito