Citizens of Romania and Bulgaria can work without restrictions across the European Union from Wednesday.
Transitional controls were imposed by some member states on nationals of the two countries - the poorest in the bloc - when they joined the EU in 2007.
Their rights to work and claim benefits were limited for their first seven years of EU membership.
Some in wealthier western EU nations fear mass migration from the two countries.
The controls were lifted as Greece took over the six-month rotating presidency of the EU.
BBC Europe correspondent Duncan Crawford says that it gives Greece the opportunity to influence the EU's political agenda.
Athens has been making reforms to overcome an acute debt crisis - a condition of a 240bn-euro (£200bn; $331bn) bailout that started in 2010.
The Greek crisis put the future of the eurozone and its common currency, the euro, at risk.
Latvia became the 18th country to adopt the euro on 1 January 2014.
EU commissioner Olli Rehn said joining the eurozone marked "the completion of Latvia's journey back to the political and economic heart of our continent, and that is something for all of us to celebrate".
But some Latvians have expressed scepticism over the move at a time of financial difficulties in Europe, as well sadness at giving up the lat, a potent symbol of the country's independence from the Soviet Union in the 1990s.
UK rules tightened
The authorities in Bucharest and Sofia say there will not be an exodus, and that many of those who wanted to leave the country to seek work had already done so.
Romanians and Bulgarians were able to travel to other EU states without a visa after the two countries joined the bloc.
However, nine of the 26 other member states imposed temporary restrictions on the kind of jobs they could take.
France, the Netherlands, and Belgium required that they obtain work permits.
In Britain, prospective employers had to apply for work permits and Bulgarians and Romanians migrants for an "accession worker card". Low-skilled workers were restricted to existing quota schemes in the agricultural and food processing sectors.
The UK government is tightening the benefit rules to ensure that migrants cannot claim out-of-work benefits for three months after arriving and will only qualify for support after six months if they have a genuine chance of employment.
A spokeswoman for the Romanian foreign ministry told the BBC that some of the media coverage in the UK had bordered on racism, and there had been an "outright campaign" against Romanians and Bulgarians.
"There isn't going to be an invasion of Romanians," Brandusa Predescu said.