House Majority Whip Steve Scalise was shot in hip by an unidentified gunman during the practice of a Republican baseball team at a field in Alexandria, Virginia.
Scalise, 51, is said to be in stable condition, his aides said.
A second congressman Roger Williams was also injured, but it is unclear if he was shot or injured in some other way.
The gunman was shot at by the Capitol Police. He has been apprehended and taken to a local hospital.
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About 25 senators and congressmen were practicing for more than a century old annual Congressional Baseball Game scheduled for June 15 at Nationals Park when the shooting took place.
According to a senator, some 50 shots rang out in the exchange of fire between the gunman and the lawmakers' security detail.
The suspect was a white man with dark hair, apparently in his 40s or 50s, wearing jeans and a blue shirt, he said.
The shooting that lasted about 10 minutes with Congressmen being the target has sent shocked waves across the country.
President Donald Trump said he was closely monitoring the developments in Virginia, and that Scalise was expected to recover.
"The Vice President and I are aware of the shooting incident in Virginia and are monitoring developments closely," Trump said.
"We are deeply saddened by this tragedy. Our thoughts and prayers are with the members of Congress, their staffs, Capitol Police, first responders, and all others affected," Trump said in a statement.
"Without the Capitol Hill police it would have been a massacre," he told Fox News.
"Capitol Police showed tremendous bravery today. I'm grateful for them and cannot thank them enough for their service," said Congressman Tom MacArthur.
Congressman Ron DeSantis, who left just before the shooting, said as he walked to his car, a man asked him if it was Republicans or Democrats practicing. About three minutes later, at around 7:15 am the shooting began, DeSantis said.
Indian-American Congressman Raja Krishnamoorthi described it as a horrible sense of violence.
"This was a horrible, senseless act of violence, and we must bring the perpetrator to justice," Krishnamoorthi said.