Over a thousand people formed a human chain around the Hungarian parliament building on Monday in protest at a government shake-up of the country's electoral system.
The protest was led by former prime minister Ferenc Gyurcsany, who says proposed legislation requiring voters to register no later than 15 days before election day could reduce turnout by "hundreds of thousands or even millions" of people.
Opponents of the proposal say such an outcome could strongly favour the conservative government coalition led by Prime Minister Viktor Orban's Fidesz party at the next elections in 2014.
"Around six million Hungarians participated in the last elections. Now there is a big chance that less than four million will vote," Gyurcsany, who was the country's prime minister between 2004 and 2009 and now heads a small opposition party, told a news conference before the demonstration.
"Around 25 percent of voters who finally vote decide to do so only in the last days and weeks before the election. These are people the government want to exclude as they tend to vote against the government as a protest," he said.
The government argues the new law would streamline a system which had required some voters to register, for example Hungarians living abroad and members of minorities within Hungary, while others did not have to.
Protestors held placards reading "We demand free and fair elections," and "Viktor, you are cheating us".
Gyurcsany said after the protest that if the bill passes parliament in a vote set for next Monday, his Democratic Coalition party will ask the European Commission and the Venice Commission to assess whether the legislation infringes European law.