Poland play a World Cup qualifier against Albania on home turf in Warsaw on Thursday.
Three days later the Poles will take on San Marino in their next qualifying fixture in Serravalle.
On September 8, Poland will play England at home in Warsaw.
Manager Paulo Sousa earlier this month released a list of 26 players called up for the three games.
Since then several players, including Dawid Kownacki, Kacper Kozłowski, Krzysztof Piątek and Sebastian Szymański, have succumbed to injuries, and one, Mateusz Klich, has been ruled out after testing positive for the coronavirus.
Meanwhile, Red Bull Salzburg defender Kamil Piątkowski and Lech Poznań winger Jakub Kamiński have been called up as replacements, alongside Legia Warsaw midfielder Bartosz Ślisz and AEK Athens player Damian Szymański.
On Monday evening, a group of 21 Polish players kicked off the camp with a training session in driving rain in Warsaw, state news agency PAP reported.
The Polish team opened its 2022 World Cup qualifying campaign with a 3-3 away draw with Hungary in Budapest on March 25.
The Poles, captained by Robert Lewandowski, FIFA's world player of the year for 2020, beat Andorra 3-0 in their second World Cup qualifier at home on March 28.
Poland in late March lost 1-2 to England in their third 2022 World Cup qualifier in London.
The qualification stage for the 2022 World Cup will run until November 2021, while the tournament will be held in Qatar from November 21 to December 18, 2022.
(gs)
Source: PAP