By Lars Rischke and Wojciech Zurawski

ZITTAU, Germany (Reuters) - Frontiers in east Europe once guarded by machineguns and barbed wire in the Cold War fell away on Friday as nine mostly former communist states joined the EU's border-free zone amid fireworks, cheers and music.

German Chancellor Angela Merkel, herself from ex-communist East Germany, hailed as historic a move seen by many as a final lifting of the old Iron Curtain.

From midnight, the nine joined 15 existing members to create an area one third the size of the United States, allowing passport-free travel for 4,000 km (2,500 miles) from Estonia to Portugal.

The extension of the European Union's so-called Schengen zone brought in Poland, Slovakia, Hungary, the Czech Republic, Slovenia, Malta, Latvia, Lithuania and Estonia.

The move is expected to boost business and tourism, though some worry about a rise in crime or illegal immigration.

Border posts were ceremonially lifted or cut, border guards left their booths and people walked freely across frontiers that once divided the former Soviet bloc from the West.

"We are very pleased to be able to experience this genuinely historic moment," Merkel said at a ceremony in Zittau on the German border with Poland, noting the borders had caused much suffering in the past.

New Polish Prime Minister Donald Tusk said the new measure was heart-warming.

People at border posts the length and breadth of eastern Europe celebrated with fireworks, cheers and music from midnight as the European Union's so-called Schengen zone was expanded.

In the German town of Frankfurt on Oder on the Polish border, one of the most politically significant frontiers in Europe with a past of war, about 2,000 people celebrated with the EU's anthem, Beethoven's "Ode to Joy," and fireworks.

"It is very good. There are no borders, so there is equality. People can communicate now and travel from one place to another without any controls," said Polish student Mikhalina Yszczak, 23, shortly after midnight.

Frontiers also fell away between the Baltic states, including Latvian-Estonian Valga-Valka where a main street had been split by a border. At the Slovak-Austrian Petrzalka/Bergen crossing, people got souvenir stamps in their passports.

"There were soldiers with machineguns here and concrete blocks which even a tank could not run over. Not even a mouse could sneak in," said pensioner Kolomam Prekop.

SOME FEARS REMAIN

The move to expand Schengen, named after a Luxembourg village where a first agreement on passport-free travel was struck in 1985, has aroused fears of increased crime or that the EU will be less secure against illegal immigration.

In Austria, the village of Deutschkreutz near Hungary hired a private security firm to patrol its streets.

Outside the EU, some in Ukraine, Belarus and Russia fear a "Fortress Europe" that will make travel more difficult, though European officials say this will not be the case.

The expansion of the Schengen zone will mean it covers 24 countries or about 400 million people. It initially covers land and sea borders but will be extended to airports next March.

The eastward enlargement of the EU in 2004 has already meant travel across borders has become much simpler.

Thousands of people from countries like Latvia, Lithuania and Poland have gone to work in Britain and Ireland, which opened their markets to workers from the new EU countries. Britain and Ireland themselves have remained outside Schengen.

Cyprus, also in the EU from 2004, has asked for a year's delay before opening its borders. Romania and Bulgaria, which became EU members this year, have yet to meet security criteria.

(Writing by Patrick Lannin in Riga, reporting by Martin Dokoupil in Bratislava, Karin Strohecker in Vienna, David Mardiste in Valka; Editing by Charles Dick)