EU and Romanian leaders exchanged political warnings at the official launch ceremony of Romania's six-month EU presidency in Bucharest on Thursday (11 January).

The barbed exchange arose amid concerns over the Romanian government's plan to overhaul the judiciary.

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Bucharest's Athenaeum hosted EU and Romanian officials for the ceremony (Photo: European Commission)

EU Council president Donald Tusk warned his hosts not to diverge from EU rules.

"To those in the European Union who might think that playing outside the agreed rules and cutting corners means strength, I say: 'You are wrong'," Tusk told Romanian leaders and EU commissioners in Bucharest's Athenaeum concert hall.

"It means weakness. To those working hard to defend European values, our freedoms and rights, I say: keep fighting," Tusk said.

The stakes for the Romanian presidency were "nothing less than the way we envisage our European future together", he added.

The EU council chief delivered his speech in Romanian, earning a standing ovation.

He also warned the against infighting in the Romanian political class.

"It depends only on you, whether, for Europe, Romanian politics will be a good example or a dire warning," Tusk told his audience.

For his part, European Commission president Jean-Claude Juncker said there cannot be any compromises on rule of law, human rights, and the fight against corruption.

He spoke came as Romania takes over the EU's rotating presidency for the first time since it joined the bloc in 2007.

Its chairmanship comes at a sensitive time, amid Brexit, EU budget haggling, and the European Parliament elections in May.

The EU has voiced concern after Bucharest fired its top anti-corruption prosecutor and pushed through a judicial overhaul, including plans to grant amnesty to politicians who had been sentenced for graft.

The Romanian government says it is tackling official abuses, but critics say the changes serve to protect corrupt politicians, such as Liviu Dragnea, the powerful head of the ruling PSD party, who is barred from holding office due to a past conviction for electoral fraud.

Thursday's prickly meeting highlighted thinking that Romania could join Hungary and Poland in an EU monitoring system on rule of law, which includes potential sanctions.

It also spotlighted the infighting between the Romanian government, fronted by prime minister Viorica Dancila, and the Romanian president, Klaus Iohannis, from the opposition National Liberal party.

Dragnea, Romania's de facto leader, did not make a speech at Thursday's EU ceremony.

But the head of the senate, Calin Popescu-Tariceanu told the EU audience that Romania's judicial reforms were needed to purge old communist elements in the judiciary, echoing Hungary and Poland's line.

"As a remnant of the communist past, some institutions and decision makers are still holding on to the unchecked power they have previously indulged in," Popescu-Tariceanu said.

These "actors" have kept their old "habits" and claimed "the same unaccountability they collectively enjoyed before 1989," he added, referring to the bloody revolution which brought down Nicolae Ceausescu, the country's late communist dictator.

Popescu-Tariceanu said the EU had "misconceptions about Romanian public life".

He also claimed the EU's founding fathers had wanted a bloc "free from any form of dictate politics" and "bureaucratic arrogance" by EU institutions.

As he spoke, hundreds of people gathered outside the majestic Athenaeum hall to protest against government corruption.

They waved EU and Romanian flags in the cold and snowy Romanian capital, while inside an orchestra, composed of musicians from each of the EU's 28 countries played Beethoven's Ode to Joy - the EU anthem.