An injured police officer has been hailed after preventing a potential massacre in the baseball practice shooting that left top US politician Steve Scalise in a critical condition.

David Bailey threw out the ceremonial first pitch on crutches at the charity Congressional game that Mr Scalise, the House of Representatives majority whip, had been due to play in.

Bailey, who injured his ankle during the shooting while returning fire on shooter James Hodgkinson, received huge applause as he walked towards the pitcher's mound.

Republican members of Congress played their Democratic colleagues in the charity game on Thursday night at Washington's Nationals Park in front of thousands of spectators.

Several politicians wore hats to honour Mr Scalise, whose drew a standing ovation throughout the stadium when his name was read out.

Mr Trump, in a video message played at the game, said the event had "a much deeper level of meaning" because of the shooting.

"In Washington we have our disagreements, but we all agree that we are here to serve this nation we love and the people who call it home," Mr Trump said. "That's the source of unity and more than ever we must embrace it."

The Republican politician's condition remains critical though hospital staff said the 51-year-old is improving after a second round of surgery.

He suffered injuries to internal organs, broken bones and severe bleeding after being shot in the left hip on a baseball field where he and other politicians were practising for the charity game.

A congressional aide and a lobbyist were also injured in the shooting.

Attacker Hodgkinson had expressed anger towards US President Donald Trump before opening fire at the early morning practice in the Washington suburb of Alexandria, Virginia.

The 66-year-old gunman was arrested before dying of his injuries.