Around 5,000 Polish youths wearing military uniform and carrying Kalashnikov replicas marched through Warsaw Thursday, pledging to come to their country's aid if necessary amid concern over Russian activity in neighbouring Ukraine.

The unprecedented rally took place 76 years to the day after the Soviet Union invaded eastern Poland at the start of World War II -- just weeks after Nazi Germany attacked the country from the west.

"Peace is never a given once and for all," said Karolina Ratajczak of the national Federation of Pro-Defence Organisations, which has around 10,000 members.

"We should always be ready to repel an attack," she told AFP in the crowd of teenagers.

Defence Minister Tomasz Siemoniak was also there. His ministry began providing the paramilitary organisations funding and equipment after security fears swept through the country over Russia's Crimea annexation and backing of pro-Moscow separatists in Ukraine.

"Poland is facing two big threats: Russia in the east and Islamist terrorism in the south," said Marcin Waszczuk, commander-in-chief of the Strzelec paramilitary organisation.

He estimates there are between 400,000 and 600,000 paramilitaries in Poland, adding that "these people could constitute a second army in the case of conflict."

A central European EU member of 38 million people, NATO-member Poland has about 100,000 professional military personnel and 20,000 reservists.