BUDAPEST (Reuters) - Thousands of Hungarians rallied against Prime Minister Viktor Orban's government on Sunday, venting anger over allegations of corruption and a secretive nuclear deal with Russia, Hungary's former communist overlord.The rally by civil groups and some leftist parties coincided with an established national holiday and with a new Sunday trading ban that forced most shops to close, despite criticism from the retail sector and an Ipsos survey showing more than two-thirds of people asked were against it.Protesters expressed disappointment over Orban's track record. While his fans hail him as a champion of national sovereignty, his opponents accuse him of a power grab and filling key public sector posts with party loyalists."There may have been mistakes over the past 20-25 years, but the country was definitely on the right track," said Jozsef Banizs, a sales manager in his fifties."But now, we have made an about-turn and are headed towards a system that we left behind in 1990," with the first free elections after the collapse of communism, he said.The groups that organized the rally on Facebook have called for a referendum on a clampdown on corruption, more transparency in public investments and for details of a 10-billion-euro ($10.5 billion) nuclear deal with Russia to be made public.The crowd, some bearing Hungarian and EU flags and banners of leftist groups, broke into chants against Orban and his Fidesz party several times while marching through Budapest.Earlier Orban said in a speech to mark the anniversary of a 19th century uprising against Hapsburg rule that Hungary should continue to stand up for its rights within the European Union.Economic growth rose to 3.6 percent last year, while Hungary has dodged a costly bullet with a last-minute conversion of Swiss franc mortgages into forints before the franc's surge.Orban's Fidesz is still the most popular party among Hungarians but its support has plunged recently and last month it lost its two-thirds parliamentary majority."Corruption has permeated every facet of the country, from business life to politics," said Robert Nagy, a 55-year-old engineer who attended the protest with his wife.Nagy said his brother-in-law, a local government employee, stayed away from the rally for fears of being reprimanded."Many people think like that in this country and a country where many people think like that is not free," he said.($1 = 0.9543 euros)(Additional reporting by Krisztina Fenyo; Editing by Ruth Pitchford)Join the conversation about this story Click here to read full news..