BUCHAREST/SOFIA – Millions of Romanians and Bulgarians revelled in their first day as citizens of the European Union Monday, after a night of fireworks and street parties celebrating their countries' entry into the bloc.

Deemed too politically and economically backward for membership during the EU's first eastward expansion in 2004, the Black Sea neighbors were relieved to join in what political analysts say was the last enlargement this decade.

The accession of the poor, ex-communist duo raises the EU's membership to 27 states, almost half of them former eastern bloc countries cut off from the West by the Iron Curtain until 1989.

"Bulgaria's and Romania's accession to the EU completes our historic fifth round of enlargement, which peacefully reunified Western and Eastern Europe," EU President Jose Manuel Barroso said in a statement of congratulations.

Romania – the larger of the two – and Bulgaria will together boost the EU's population by 30 million, to 490 million, but will add just 1 percent to its economic output.

Once ruled by two of the Cold War's most hard-line regimes, the Danube pair stretch the EU's borders from the Atlantic and Baltic in the west and north to the Black Sea in the southeast.

Their entry was marred by foot-dragging in Bucharest and Sofia on fighting graft and organised crime, exacerbating worry in some EU states that the bloc may have spread itself too far.

Fearing new waves of immigration – like one that followed the 2004 expansion – and crime could drive citizens out of jobs and undermine society, some EU capitals want accession hopefuls Turkey and the western Balkans to wait much longer to enter.

Braving freezing temperatures, groups of Romanians and Bulgarians crossed newly opened borders shortly after midnight to visit neighboring towns in Hungary and Greece, and across the Danube, until now kept distant by lengthy passport controls.

In the Romanian port of Sulina, nicknamed 'Europolis' in the 19th century, ships sounded sirens for half an hour and people danced along the Danube where it empties into the Black Sea.

"It was a difficult road ... Now we are taking a new path, a path to victory and happiness," Romanian president Traian Basescu told a crowd of 40,000 in Bucharest.

In Sofia, EU Enlargement Commissioner Olli Rehn said membership meant Bulgaria could depend on full EU solidarity as it tries to free five Bulgarian nurses sentenced to death in Libya for infecting hundreds of children with HIV. Bulgaria and its allies say the verdicts are a miscarriage of justice.

"You have our full support," Rehn told packed concert hall in Sofia. "You are not alone. You are with the European Union."

Eager to tap 40 billion euros ($53 billion) in EU funds to overhaul dilapidated infrastructure and boost industry, the newcomers hope to close a yawning wealth gap with the West.

But problems remain. Their economies are growing fast, but per capita income is a third of the EU average and many observers fear incompetence can curb accession benefits.

Despite efforts to tackle corrupt "big fish" politicians, Romania also is by far the EU's most corrupt country, ranking 84 out 163 countries on Transparency International's corruption perceptions index, behind China and Cuba.

And in Bulgaria, powerful organised crime gangs born from Soviet-era secret services control large parts of the economy, diplomats say, and authorities have failed to convict a single suspect for over 150 gangland murders since 2001.

Some diplomats fear that, having achieved admission, politicians may relax on reforms and Brussels has vowed to penalise the new members if they fall behind.

But Bulgarians and Romanians hoped their new status would end to prejudice they felt as EU outsiders.

"Until today when I have gone to other countries, every waitress, every salesman turned up his nose when hearing I come from Romania," said salesman Sergiu Radu, 27.

"I hope this means an end to that shame and frustration."