Blue-and-gold European Union flags fluttered across the Romanian capital and celebratory fireworks thundered through the sky as Romania, along with southern neighbour Bulgaria, became the latest countries to join the European Union on Monday.

"It was hard but we arrived at the end of the road. It is the road of our future. It is the road of our joy," Romanian President Traian Basescu said to cheers from a crowd of tens of thousands of revellers packed into University Square.

"We arrived in Europe. Welcome to Europe," Basescu said from a stage, where he was joined by EU Enlargement Commissioner Olli Rehn, government ministers and European foreign ministers.

"Citizens of Bucharest. Welcome to the EU," Rehn said in Romanian and English.

Expansion, immigration concerns

Romania and Bulgaria — two much poorer countries than the rest of the EU countries — bring 30 million new members to the union and expand the number of member countries in the bloc to 27.

Their entrance into the union has sparked concerns over continued expansion eastward, as well as fears among more wealthy member states of a flood of immigrants seeking work and higher wages than in their home countries.

The foreign ministers of Germany, Denmark, Austria and Hungary wished Romanian citizens a Happy New Year. Officials were due to fly later Monday to Bulgaria for celebrations.

"Entering the European Union, we are assured peace and prosperity. This is an enormous chance for new generations," Basescu said as the clock ticked down to midnight.

He later waved a huge Romanian flag and said he hoped Romanians would retain their national identity.