A man who allegedly wrestled a gun from a police officer and opened fire while under guard at a hospital in Canberra has been charged with attempted murder.

The man is accused of assaulting police about 4pm on Wednesday, taking one of their weapons and firing it as he tried to escape.

He was soon restrained by police, and no-one was injured during the incident.

ACT Policing say the man has now been charged with two counts of attempted murder, assault and attempting to escape custody.

The 26-year-old was being held at Canberra Hospital after being arrested over a serious traffic incident on Wednesday morning.

Witnesses told the Nine Network they heard shouts of “Code Black!” as the hospital was put into lockdown.

“My wife couldn’t get in to see her little boy for about an hour and a half. It was a little bit (scary),” said Dane O’Connell, who was in the emergency department with his young son.

“I couldn’t do anything really … police were everywhere.

“My phone was flat, so I couldn’t even call my wife to know what was going on.”

Police have launched a critical incident investigation, but say a preliminary assessment has found no misconduct by the officers involved.

-AAP