In March, Poland adopted a new law on homeland defence, which introduced a one-year-long military service on a voluntary basis. The defence ministry started the recruitment for the service in late May, with military picnics and 70 recruitment offices in cities and towns around the country.

At a Sunday swearing-in ceremony in Nowy Dwór Mazowiecki, central Poland, Mariusz Błaszczak, the Polish defence minister and deputy prime minister, said Poland's duty was to build up the Polish military so that the country could respond to the current threats.

"There is a war going on across our eastern border, just like in the images from World War II. There are war crimes, civilians are murdered," he said.

"The goal of our country's authorities is to strengthen the Polish armed forces so that Poland, through a strong army, can effectively deter the aggressor, so that the Kremlin rulers do not dare to attack our country," Błaszczak said.

He said that around 8,000 candidates have signed up for voluntary military service so far.

Błaszczak added that Poland's "minimum aim" is to increase the number of soldiers in the Polish armed forces to 300,000, including 250,000 professional soldiers and 50,000 Territorial Defence Force (WOT) volunteers.

Poland suspended mandatory conscription in 2010 in order to professionalise its armed forces.

According to the government, Poland now has about 111,500 professional soldiers and 32,000 WOT troops.

Those who want to join the military have three options now. They can become professional soldiers, decide to do basic military service or join the territorial defence force.