Two surgeons who were having lunch with their families near Krakow's Main Market Square ran into the aftermath of Saturday's mass shooting and treated the wounded on the street, with one of them almost certainly saving the life of a police officer shot in the chest.

Professor Michal Pedziwiatr and Dr Mateusz Wierdak had just finished lunch at a nearby restaurant when gunfire rang out across the crowded square shortly before 3pm on Saturday. Pedziwiatr told Polish media that his son shouted a warning as people began falling to the ground. While their families took refuge in a nearby pharmacy, the two doctors ran towards the scene.

When the shooting stopped, they found two police officers lying on the ground, conscious but with gunshot wounds to the chest. The gunman was also conscious, with wounds to his abdomen, thigh and chest. Wierdak went straight to Senior Constable Jakub Kulik, who was bleeding heavily from a chest wound, and used his hands and bandages to stem the flow of blood before emergency crews arrived.

"If not for this, the officer probably wouldn't have survived, as he would have bled to death," Pedziwiatr said. He then began assessing the other seriously wounded people on the street, before later operating on victims of the attack in surgery.

Kulik, who was shot three times, had used his last remaining strength to subdue the attacker and then knelt guarding him until he collapsed. A former soldier with the elite 6th Airborne Brigade, he had only recently left the military to join the police. Both officers were later described as stable after surgery, and the city said it would nominate them for decorations and support their families.

Pedziwiatr, a professor of medicine at Krakow's Jagiellonian University, played down the pair's actions. "We don't feel like heroes, but we did everything a paramedic would do at the scene of such an incident," he said.

Six people were killed in the attack, including a 32-year-old British national, and 16 others were wounded, among them foreign citizens from the UK, Austria, Germany, Portugal and Russia. The gunman, named in the Polish press as Michal Oksinski, 24, had driven around 200 kilometres from his home in Ostrowiec Swietokrzyski before opening fire at random from his car. Police said the first patrol arrived 23 seconds after the attack was reported and the suspect was shot and neutralised within three minutes. An AR-15-type assault rifle was recovered, and investigators are examining other weapons found in his vehicle.

Hours later, the bodies of three people identified as the suspect's 53-year-old father, 52-year-old mother and 21-year-old sister were found at his home. Prosecutors have opened a terrorism investigation, and Prime Minister Donald Tusk said Poland had never seen such a drastic case of terrorism. Poland is observing two days of national mourning, and the traditional bugle call from St Mary's church was suspended for 12 hours, the first time it has fallen silent since 1939, before returning on Sunday morning.