LONDON, England (CNN) -- The nine newest members of the European Union were celebrating on Friday after border controls officially were lifted, allowing them to move freely within the EU's passport-free zone.
At midnight Thursday, the EU expanded the zone to embrace the eastern and central European countries that joined in 2004 -- Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia.
The nine, including three former Soviet republics, joined 15 already enjoying the benefits under the Schengen agreement. See a map of the passport-free zones »
European Commission President Josi Manuel Barroso welcomed the extension of the Schengen zone, which will make it possible to travel 2,500 miles from Estonia to Portugal without having to show a passport.
"Together we have overcome border controls as man-made obstacles to peace, freedom and unity in Europe, while creating the conditions for increased security," Barroso said.
The occasion was marked by special border ceremonies, including one on Germany's eastern border with Poland and the Czech Republic at Zittau. Barroso attended along with luminaries including German Chancellor Angela Merkel and Polish Prime Minister Donald Tusk.
Despite the celebrations, the expansion has prompted security fears that illegal immigrants, drug smugglers or terror suspects could find it easier to cross into Europe from its eastern borders.
To counter these concerns, the European Commission has spent 1 billion euros ($1.5 billion) on security improvements along its eastern fringes, including thermal imaging to track illegal migrants.
The U.N.'s refugee agency said on Friday that there has been a sharp increase in asylum seekers crossing into eastern Europe in recent months.
Since July, Poland has seen a steep increase in the number of asylum seekers crossing into the country, many from Chechnya and other ex-Soviet states, the agency said in a statement.
So far in 2007, there have been nearly 5,000 asylum claims in Poland, over 3,500 of which came after June. Around 1,150 of the claims came in November alone, officials said.
The agency said there were reports that smugglers were spreading rumors that the borders would be closed to asylum seekers under the new arrangements, prompting the influx and leading to overcrowding at centers that house migrants in eastern Poland.
In addition, Switzerland has signed up to the Schengen agreement and is set to implement it next year. Britain and Ireland are the only EU countries to have so far opted out.
Although the new rules mean travelers can move freely without a passport, they can still be asked to carry documents by any of the countries concerned.
For non-EU nationals, a Schengen visa will allow travel across all the participating countries. E-mail to a friend
CNN's Charles Hodson and Robin Oakley in London contributed to this report