US Capitol Police (USCP) arrested at least four people during the Saturday rally, one of whom was said to have been carrying a firearm.
Someone in the crowd reportedly spotted the man carrying a handgun and told USCP officers.
Two more people were detained at Louisiana Ave for felony extraditable warrants out of Texas, and one more was arrested for a probation violation.
Meanwhile, footage circulating on social media suggests the armed man might have been an undercover law enforcement officer.
Some 700 people were expected to participate in the rally, according to a US Department of Homeland Security forecast. However, the USCP reported between 400 and 450 people at the march.
It was difficult to tell the real number due to the presence of reporters and counterprotesters from various movements, including Antifa, as seen in pictures from the rally.
Counterprotesters at the Justice for J6 rally in support of people arrested after the January 6 riot
© Sputnik
People gathered in DC on September 18 to participate in the Justice for J6 rally in support of people arrested after the January 6 riot
© Sputnik
At least 100 police officers were deployed at the square, with a helicopter circling over the US Capitol. A law enforcement spokesperson told Sputnik that there was no threat of organized riots and violence.
DC officials earlier said that over the last several weeks, they prepared contingency plans designed to immediately defuse any violence that may erupt on Saturday.
People gathered in DC on September 18 to participate in the Justice for J6 rally in support of people arrested after the January 6 riot
© Sputnik
The rally, organized by the Look Ahead America group supporting former President Donald Trump, intends to bring public attention and support to some 600 arrested, kept in detention and charged with crimes in relation to the January 6 events at the Capitol.