* Protesters burn tires, block roads; businesses vandalized * Union boss calls mass work stoppage over tax policy * Workers protest government's response to high inflation By Alejandro Lifschitz and Nicolás Misculin BUENOS AIRES, Nov 20 (Reuters) - Opposition trade unions protesting Argentina's economic policies brought public transportation and the country's crucial grain exports to a halt on Tuesday in the first general strike since President Cristina Fernandez took office five years ago. Demonstrators burned tires to block roads and vandalized a handful of the businesses that opened despite the 24-hour work stoppage called by bus drivers, train conductors and port, airline and bank workers. They rallied in places including Plaza de Mayo in front of the presidential palace in Buenos Aires. The general strike - the first to hit Argentina in a decade - follows broad protests held on Nov. 8 over high crime, soaring inflation and the government's policy response. Farmers also joined the protest led by Hugo Moyano, a gruff former truck driver once closely linked to Fernandez but now a leading opposition figure. He wants lower taxes for workers whose purchasing power has been drained by galloping inflation. "The silence of the streets, the absence of people in the streets, in the shops, in the businesses - this is the voice that the government must hear," Moyano told reporters, vowing to keep pressing the demands of his CGT labor federation. Fernandez's popularity has tumbled since she easily won re-election last year. Inflation is running at about 25 percent despite a sputtering economy, according to private economists. The government publishes much lower inflation data long dismissed by the markets as inaccurate. The strike increases the stakes in the political battle between Moyano and Fernandez, who condemned the strike and said she would not be swayed on policy. "Today wasn't a strike. It wasn't even a picket. This was about strong-arm tactics and threats," she told supporters at a rally. "We cannot bow to extortion." Ties between Moyano and the president soured after the death in late 2010 of Nestor Kirchner, Fernandez's husband and predecessor as president. Moyano's CGT split earlier this year, with his allies re-electing him as leader in a vote rejected by rival union bosses aligned with Fernandez. The fracture in the umbrella group risks deepening labor unrest as inflation stokes wage demands. 'LOSING CONTROL' "This general strike raises the possibility that she is losing control of the street and it puts the unions that are allied with her in an uncomfortable position," Ignacio Labaqui, who analyzes Argentina for emerging markets consultancy Medley Global Advisors. Fernandez, meanwhile, is moving to shore up her base. Her allies in Congress last month lowered Argentina's voting age to 16 from 18, a change that could help the politically ailing president court the youth vote ahead of 2013 mid-term elections. Argentina is the world's top exporter of soy oil, needed to make biofuels, and soymeal used to feed cattle as far away as China, where the emerging middle class is clamoring for beef steak. The South American country is also the second-biggest corn exporter after the United States. "This (strike) was necessary, unfortunately," said Eduardo Buzzi, who heads the Argentine Agrarian Federation, which represents small-scale farms. "There is no way to dialogue. ... This is the most anti-farm government Argentina has ever had." The agricultural sector has long quarreled with Fernandez over the 35 percent export tax her government puts on soybean exports and curbs it places on corn and wheat shipments. Telephone calls went unanswered at the main grains port of Rosario. The usually noisy, truck-jammed entrance to the port of Buenos Aires was still, with activity expected to resume on Wednesday. The local stock and bonds market was also quiet. Just up the street from the presidential palace, tourist attraction Cafe Tortoni closed its doors after being vandalized. "It's painful," a cafe employee told state television. "This is like a second home to us."