The fight broke out around noon after pro-Trump supporters and anti-Trump protesters clashed at the rally. "We're here to support the American veterans, our military, our law enforcement, our first responders, our president and vice president", Jennifer Sterling said.
The attack, which led to three arrests, comes after the president's tweet thanking people for rallies.
Travis Guenther, whose wife was also pepper-sprayed, said he was among those who chased the masked man and struck him with a flag that said, "Trump, Make America Great Again".
A fistfight erupted between pro-Trump demonstrators and counter-protesters on the sands of a Southern California beach on Saturday, while Philadelphia police were looking into an officer's use of force in that city as "Make America Great Again" rallies were held across the country. Approximately 2,000 marchers and 30 counter-protesters showed up at the rally at Bolsa Chica State Beach.
Kevin Pearsall of California State Parks Police says the arrests happened when counter-protesters sprayed pro-Trump supporters.
The Los Angeles Times reported that the black-clad Trump protester armed with pepper spray was also wearing a mask.
Demonstrators in Huntington Beach were met by counter-protestors, exchanging blows and pepper spray, video from demonstrators showed. Not all of them ended in violence, but many were disrupted by masked protesters, who bullied and assaulted Trump supporters.
Tensions escalated between the groups when ten, mostly masked, individuals created a human wall to try and stop the march.
Demonstrations denouncing the Trump administration have drawn hundreds of thousands since he took office in January.
Fearing clashes between pro-Trump and anti-Trump crowds, the event was later moved to a different location, CBS Los Angeles noted.