Romania's Constitutional Court ruled Tuesday that a referendum to impeach the country's president was invalid, leaving Traian Basescu in power after months of political turmoil.

"The court noted that the turnout was lower than the minimum threshold required for the ballot to be valid," the judges said.

"The ruling was adopted with a qualified majority," one of the nine judges, Tudorel Toader, told reporters at the end of a nearly three-hour-long meeting.

Basescu, who has been suspended from the presidency, is now expected to return to his office to see out the rest of his second five-year term which started in 2009.

Interim president Crin Antonescu announced he would make a public address shortly.

Romania's ruling centre-left coalition USL headed by Prime Minister Victor Ponta last month launched a move to impeach centre-right rival Basescu, which led to a referendum that failed as voter turnout fell short of the required 50 percent threshold.

In the referendum an overwhelming majority of voters -- 87 percent -- supported ousting Basescu, so the USL contested the electoral rolls after the ballot, forcing the top court to rule on its validity.

On Monday, Ponta's government sent the court new voters' numbers, hoping this would overturn the result.

Basescu's allies slammed the manoeuvre as "illegal", while the media said the new figures were aimed at "sowing confusion."

"I hope the ruling will put an end to the political crisis and the politicians who started the impeachment move have learnt their lesson," former centre-right prime minister Emil Boc said.

He added that he hoped Basescu and the centre-left coalition would work together in an "institutional and constitutional way."

But a member of the USL, Puiu Hasoti, decried the ruling as "anti-national" and warned it would only deepen the crisis.

"Romania has proven to have strong institutions - the Constitutional Court and the Prosecutor's Office - to name just two," Laura Stefan, an anti-corruption expert of Forum Expert think tank, told AFP.

The ruling, adopted by the court despite "tremendous political pressure" on its members, will be a landmark in Romania's efforts to consolidate the rule of law, she stressed.

Some 200 demonstrators meanwhile gathered in front of the court's headquarters calling for Basescu to be ousted.

The organisers of the protest, members of a reservists' trade union favourable to the USL, said they would stage rallies daily and "wreak chaos" in Romania if the court invalidated the referendum.

The European Commission and the United States have voiced deep concern over the ruling coalition's controversial moves to impeach Basescu which they said threatened the rule of law.

The power struggle between the USL and Basescu has sunk the local currency and rattled foreign investors, putting economic recovery at risk.

Last week the International Monetary Fund and the EU cut back their growth forecast for this year from 1.5 percent to 0.9 percent, citing the eurozone crisis, the political turmoil in Bucharest and the effects of drought.