CARACAS, Venezuela - Venezuelan President Hugo Chavez threatened Saturday to halt oil exports to the United States and said opponents of his leftist government are not welcome within the military or the state-run oil company. Also on Saturday, tens of thousands of supporters of Manuel Rosales, Chavez's main challenger in Dec. 3 presidential elections, staged a 16-mile march through the capital Caracas. More than 1,000 police were deployed along the route to prevent clashes between Rosales supporters and "Chavistas" who gathered on street corners shouting "Viva Chavez!" and "Oh, No! Chavez Won't Go!" as the marchers passed. The opposition has accused Chavez's administration of political coercion after Oil Minister Rafael Ramirez was caught on videotape threatening to fire employees of state-run oil company Petroleos de Venezuela SA, or PDVSA, who oppose Chavez. "If they try to destabilize PDVSA, if the empire and its lackeys in Venezuela attempt another coup, ignore the outcome of the elections or cause election or oil-related upheaval, we won't send another drop of oil to the United States," Chavez said in a speech to PDVSA workers in the coastal city of Puerto La Cruz, 150 miles east of Caracas. Chavez — a close ally of Cuban leader Fidel Castro — said President Bush "had better tie down his crazies here in Venezuela" to prevent a possible end to petroleum exports. Venezuela supplied 12 percent of U.S. crude oil imports last year and the U.S. remains the top buyer of Venezuelan oil. On Friday, Chavez suggested anyone who does not like his leftist policies should go somewhere else, like Miami. Television footage released during the week by opposition supporters showed Oil Minister Ramirez telling PDVSA workers to back Chavez or give up their jobs. Opposition leaders said it was clear proof of political coercion which violated rules against the use of state institutions as campaign tools. Earlier Saturday, Rosales urged PDVSA employees to help vote Chavez out of office. "They can be assured that nobody is ever going to know for whom they voted," he told reporters during the march through the capital. Rosales' campaign says electoral officials can impose a maximum fine of $7,800 on Ramirez if it finds campaign rules have been broken. It also is demanding an investigation by prosecutors. In comments published by the Venezuelan daily newspaper Ultimas Noticias on Friday, Ramirez said he did nothing to violate campaign rules because there was no explicit call "to vote for one candidate in particular but rather we backed President Chavez as head of state." He said more internal videos would be released to quell any doubts.