The arrest happened in a park across from Union Station.
A man with a gun has been arrested in a park near the U.S. Capitol, according to Capitol Police.
"USCP Officers just arrested a man with a gun in the park across from Union Station. At this time we have no reason to believe there is an ongoing threat. We are working to gather more information and will put out more details when they are confirmed," police said in a statement.
Ahmir Lavon Merrell, 21, from Atlanta, was taken into custody, according to Capitol Police.
The incident Tuesday was the second involving a firearms arrest near the Capitol in the last three days. On Sunday, two men were arrested after they crashed a stolen car into a barricade outside the Capitol. They were found to be in possession of two handguns during their arrest -- one of which was modified to turn the weapon into a machine gun.
Police said they were alerted Tuesday to a man with a semi-automatic weapon and responded to the park and found Merrell. Police asked him to stop, but he did not and was Tasered by a Capitol Police officer. He was later taken to a hospital for treatment after being stunned.
Authorities believe he is "dealing with some mental health issues," Capitol Police Chief Tom Manger said on Tuesday.
Police said they don't believe Merrell had a specific threat or target, however, Manger said "the fact that he was on Capitol grounds, is a concern." Police said he will preliminarily be charged with having a firearm on Capitol grounds.
Merrell is also a registered sex offender in Georgia, police said.
The park was searched and secured and the suspect's belongings were searched "out of an abundance of caution," Capitol Police said.
ABC News' Allison Pecorin and John Parkinson contributed to this report.