An American lawmaker was among several people shot in a multi-minute scene of terror that unfolded on a baseball field today, sending five people to hospital with a variety of injuries.

A shooter blasted dozens of rounds at a gathering of Republican lawmakers practicing for their annual baseball game against Democrats, spraying bullets across the field from his vantage point along the third-base line.

A prominent politician was among several people hit: senior congressional Republican Steve Scalise, who fell to the ground near second base and crawled toward the outfield in search of safety.

The injuries to the Republicans' majority whip were reportedly not life-threatening. Witnesses said the tragic scene could have been unfathomably worse, if not for Capitol Hill police on the scene who exchanged fire with the gunman.

The ensuing chaos drew about 100 police officers from different departments within minutes, according to witnesses, and helicopters swooped in to airlift the injured from the baseball field in suburban Alexandria, Va., outside Washington.

The suspected shooter was arrested. He is among the five injured, police said.

"Our lives were saved by the Capitol Hill police. Had they not been there it would've been a massacre," Sen. Rand Paul told Fox News of the scene he witnessed.

"You are completely helpless. Having no self-defence and no way to get to somebody. The field was basically a killing field. If you were to run out there while the shooter was still shooting."

He told another interviewer that everyone there faced a life-and-death, split-second dilemma: Stay where they were, or scurry for safety in the outfield or the dugout and risk becoming easy targets in an open field.

Another Republican described how people fled to the dugout. They huddled in terror, not knowing whether the gunman might move in their direction and pin them down in an enclosed area.

"We didn't know if there were other shooters that had us surrounded and would come into the dugout," Arizona Sen. Jeff Flake told reporters on the scene.

"We didn't know whether to run."

Congressman Mo Brooks said he was on deck, preparing to bat, when the shooting started. He saw a rifle near third base. Then he said he suddenly saw Scalise bleeding, crawling into the outfield.

He said there were easily 50 shots fired.

He said Scalise, 51, suffered "a hip wound." The Alabama lawmaker said his colleague left a trail of blood as he crawled away: "We started giving him the liquids, I put pressure on his wound in his hip."

House Speaker Paul Ryan's office said Scalise's wounds were not believed to be life-threatening and that a member of the security detail was also shot. Scalise is the No. 3 House Republican leader. He was first elected to the House in 2008 after serving in the state legislature.

Another Republican lawmaker left a few minutes before the shooting. On the way out, he had a quick conversation he has since reported to police.

Ron DeSantis said he was walking to car with a colleague and described to Fox News an exchange he had with a man, who was unarmed: "There was a guy that walked up to us that was asking whether it was Republicans or Democrats out there. And it was just a little odd. And then he kind of walked towards the area where all this happened. ...

"We've told the police."

Political rivals expressed their own horror at what unfolded.

Democrats preparing at their own baseball practice prayed for their Republican colleagues when news broke. California congressman Pete Aguilar tweeted: “My heart is heavy right now. We just said a prayer for our colleagues and are holding to leave Dem baseball practice."

The incident instantly brought to mind another American political shooting.

The victim of that shooting, Gabrielle Giffords, is no longer in Congress and has waged a years-long struggle to recover. The Arizona Democrat responded to the news on Twitter: "My heart is with my former colleagues, their families (and) staff."

She called the Capitol police public servants and heroes.

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