Thousands of people protested outside Hungary's parliament on Saturday calling for the ouster of Prime Minister Viktor Orban and his centre-right government which they say has undermined democratic values and weakened the economy.

Today we must say decisively and clearly, Viktor Orban and his gang must be voted out as soon as possible at an election, Peter Konya, leader of the Hungarian Solidarity Movement, told about 6,000 supporters.

The group is one of a handful of grassroots organisations that sprang up last year to fill the vacuum when the main opposition Socialists could not capitalise on the ruling party's loss of support.

It was the first demonstration in a string of such rallies, pro- and anti-government, that is customary around Hungary's March 15 national holiday, commemorating an 1848 revolution and failed war of independence against Austria.

GOVERNMENT ADAMANT

The ruling Fidesz party dismissed the rally, saying it has begun to restore economic stability and worked successfully to preserve political stability in the process.

No mass protest has formed against social reform that would force the governing powers to back down, Fidesz spokeswoman Gabriella Selmeczi wrote in an e-mailed statement.

Hungary, the most indebted nation of eastern Europe, has been at odds with the European Union over a host of issues, which has delayed the start of talks on a vital loan package with the EU and the International Monetary Fund.

Support for Orban and his party has plummeted since they were swept into power in 2010 with an unprecedented two-thirds majority in parliament.

The latest polls show backing for Fidesz at 25 percent. The next election is due to be held in 2014.

Critics say the government has weakened the system of checks and balances, such as the Constitutional Court, and expanded its powers in areas like the judiciary and media regulation.

The ruling party also pushed through a new constitution with scant consultation with opposition parties, while its unorthodox economic policies shook market confidence and contributed to the weakness of the country's forint currency.

Konya's list of grievances included a flat-tax system that he said was unfair because it cut taxes for the rich while it did not benefit the poor; as well as the sanctity of private property and legal security, which he said were lacking.

Brussels started a legal procedure against Hungary earlier this year over some laws affecting the independence of the central bank, the judicial system and data protection.

The loan talks, which Hungary has hoped would wrap up in the second quarter, are yet to start, and Orban's government has given mixed signals, promising to tackle the problems but stressing it would not back down on policies it considered central to its goals.

(Reporting by Marton Dunai; Editing by Michael Roddy)