Britain and Ireland have not joined Schengen and new EU members Bulgaria and Romania are not yet allowed in. The passport free travel is not universally supported in the member countries though.
Germany's GdP police union warned Thursday that the extension of the Schengen zone eastwards could unleash a crime wave. The lifting of border controls with Poland and the Czech Republic in particular was "an invitation to criminals," union chief Josef Scheuring said.
He said Europe's citizens will "suffer a considerable loss in terms of security".
Many Austrians also fear higher crime, according to a poll released by ORF public television poll which said 75 percent of Austrians opposed the lifting of barriers.
Similar concerns were expressed on the Slovakian side of the fence.
"It will allow more criminals to come, for example from Austria, mostly from the Turkish minority and from former Yugoslavia," said Ondrej Kralik, a Slovak policeman who has worked 11 years at various border crossing points.
In Warsaw, the head of the EU's border watchdog, Frontex, Ilkka Laitinen, warned that illegal immigration would be the price Europe paid for Schengen expansion.
Once people enter the zone, whether legally or otherwise, they would be free to move across all member states, he said.
Nevertheless, political leaders were eager to play down such fears.
Schengen "is not about criminality, it is not about insecurity or fear. It is a bigger zone of peace, security and stability," the Austrian chancellor said.
The expansion has taken years of preparation, with newcomers obliged to join the Schengen Information System (SIS), which provides police and customs officers with information about people, vehicles or goods.
The 15 older signatories to the treaty were: Austria, Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain and Sweden.