The demonstration was large but peaceful - 2000 flag-waving supporters of President Donald Trump walking and chanting along Bolsa Chica State Beach in California.
Then, they caught sight of the counterprotesters.
The melee that followed yesterday was captured in a profanity-filled video that quickly spread across the US - a violent episode among dozens of peaceful demonstrations across the country.
When the California scuffle was over, two people had minor injuries that didn't require medical attention, Captain Kevin Pearsall of the California State Parks told the Associated Press. Three people were arrested and charged with using illegal pepper spray; a fourth was charged with assault and battery.
The video detailed the violence: One man was held down to the ground and punched and hit in the face with pepper spray. Later, another man was hit over the head again and again with a "Make America Great Again" sign.
"I hit him five times with the flag over his head," said Travis Guenther, calling it retaliation because the man allegedly had used pepper spray on Guenther's wife. He could be seen yelling at the man while being detained by law enforcement officials.
A group of counterprotesters - some were from the local socialist party, according to Los Angeles ABC-affiliate KABC - decided to form a human wall, blocking the larger mass.
The video makes it hard to see what sparked individual violent incidents. At one point, two men began punching each other. A woman was hit in the face in the scuffle, further angering people. As that was going on, someone released a mist of pepper spray, and people in the dense crowd started coughing and rubbing their eyes.
Then, the mood of the rally soured, according to the video. Someone spied a counterprotester and yelled, "Get him". Two men tackled him to the ground, one throwing fists, another spraying mace.
As the counterprotesters backed cautiously away from the larger crowd, a man taunts them, "You can't run. You can't hide".
According to the Los Angeles Times, the counterdemonstrators were overwhelmed by hundreds of Trump supporters. They told a Los Angeles Times reporter that they used pepper spray only after they were shoved and punched by members of the rival group.
Despite all the violence, there were also scenes of unity, according to news reports. Some protesters and Trump supporters were seen having civil discussions after the arrest, and even hugging.