In Kraków, temporary barriers are due to be installed overnight on Tuesday on streets leading to the Main Square and the Small Market Square.
The city roads authority said the locations were agreed with police.
Retractable bollards are planned as a permanent solution, with the mayor, Łukasz Gibała, saying the first would be installed this year and more in 2027.
They are being designed with police, firefighters and ambulance crews so they do not hinder emergency services.
Gibała said talks were under way with the regional governor’s office on jointly spending PLN 6 million (nearly EUR 1.4 million) on the project.
Warsaw has placed concrete barriers at the entrances to streets leading to Krakowskie Przedmieście in the city centre and has rerouted public transport.
Mayor Rafał Trzaskowski said heightened precautions would stay in place until the causes of the attack were established.
Other cities have taken similar steps.
Wrocław, which already has nearly 80 concrete barriers, is adding more and increasing patrols, including at shopping centres.
Toruń is using municipal vehicles as mobile barriers at entrances to its Old Town.
Lublin will spend PLN 500,000 (EUR 115,00) on extra police patrols and has told operators to use all its CCTV cameras to watch for unusual activity.
Saturday's attack
The 24-year-old Polish man, who was masked, fired an automatic weapon at random people on the square on Saturday.
Six were killed and 16 injured, including two police officers and the gunman himself.
Regional governor Krzysztof Kleczar said the dead were four women and two men.
Four were Polish citizens: three women aged 47, 53 and 61, and a man aged about 50.
The others were a 51-year-old Austrian woman, who had come to Poland for the weekend with her husband, and a 32-year-old British man.
The gunman circled the square in his car, then fled towards the Kazimierz district.
After a police chase, he was shot by an officer and arrested.
He died in hospital early on Monday.
He had a firearms permit and no criminal record.
Officers found a number of weapons and ammunition in his car.
Police also discovered the bodies of his parents and sister at the family’s flat in Ostrowiec Świętokrzyski.
(ał)
Source: PAP, polskieradio24.pl, tvn24.pl