Tens of thousands of people have taken part in a controversial prayer event for the salvation of Poland and the world.

Held on the feast day marking a Christian victory over the Ottoman Turks in 1571, Catholics from more than 300 churches were bussed to 4,000 locations all along the border.

Although church leaders say the event on Saturday was purely religious, there are concerns it endorses Poland's refusal to let in Muslim refugees.

The crowds stood in lines along the border, including on the beaches of the Baltic Sea, as well as in towns and fields.

Halina Katarska, 65, said she was praying for the survival of Christianity in Europe as well as giving thanks for the survival of her son in a car crash.

'Islam wants to destroy Europe,' she told the Associated Press. 'They want to turn us away from Christianity.'

Krzysztof Januszewski, 45, said he worries Christian Europe is being threatened by Islamic extremists.

'In the past, there were raids by sultans and Turks and people of other faiths against us Christians," said Januszewski, a mechanic who traveled 350 kilometers (220 miles) to Gdansk from Czerwinsk nad Wisla,' he told Associated Press.

'Today Islam is flooding us and we are afraid of this too,' he added. 'We are afraid of terrorist threats and we are afraid of people departing from the faith.'

Poland is deeply devout with 96 per cent of the population identifying as Catholic. Alongside Hungary and the Czech Republic, the government has refused to take part in the EU's resettlement of refugees and declined to accept any Muslim migrants.

But the state's position has been criticised by church leaders and on a visit last year Pope Francis called for a greater acceptance of migrants. Polish bishops have also called for some selected Syrian refugees to be taken in.

Organisers of the prayer event deny it is directed against anyone and say the border was chosen to symbolise covering the whole world in prayer.

'We come to the border of Poland to pray for the Poles and for the whole world,' another woman told the BBC.

'We want our Catholic faith to continue, to keep our children safe, that our brothers from other countries can understand that our faith is unwavering and that we feel safer, not only in Poland but also in the world.'