(Updates prices) By Susan Mathew Aug 12 (Reuters) - Argentine stocks and the peso plummeted 30% on Monday as fears of a possible return to interventionist policies gripped after conservationist President Mauricio Macri lost by a much wider-than-expected margin to opposition in presidential primaries. The peso recovered some to trade at 52.15 per dollar, after having fallen to an all-time low of 61.99, as the central bank used $50 million in its own reserves to defend the peso. The country's main stocks index posted its worst day ever with all its components well in the red, while the cost of insuring exposure to Argentina's sovereign debt surged almost 1,000 basis points. Stumping expectations, opposition candidate Alberto Fernandez - whose running mate is former President Cristina Fernandez de Kirchner - dominated the primary on Sunday, significantly lessening Macri's chances of being re-elected in October. Concerns among market participant stem from possible policy discontinuity and intervention as experienced earlier under Kirchner's government. "What is unclear is exactly how Fernandez and Kirchner intend to conduct economic policy if they come to power," said Alejo Czerwonko emerging markets strategist at UBS Global Wealth Management's chief investment office. Concerns remain high and have to do with questions such as the composition of the economic team - who will oversee designing and implementing economic policy, what is the relationship that the new administration will have with the IMF, among others, Czerwonko said. Fernandez has said he would seek to rework Argentina's $57 billion standby agreement with the International Monetary Fund. Other worries for investors include the opposition's view that the beleaguered peso is over-valued and its call for lower interest rates on the central bank's short-term notes. While Argentine assets are expected to remain pressured for the near future, UBS's Czerwonko agrees with some analysts who say the fear of contagion to other Latin American markets and broader emerging markets remains low as Argentina's issues are "self-inflicted and very idiosyncratic." Other currencies in Latam fell between 0.5% and 2%, and most regional stocks market fell almost 1%, in line with markets worldwide. A Goldman Sachs warning that reinforced fears of a prolonged U.S.-China trade war tipping the global economy into recession, along with worsening protests in Hong Kong, and the Argentine nosedive made for a strong cocktail, sending markets into a tailspin. Brazil stocks fell 2%. The country's right-wing President Jair Bolsonaro warned on Monday that his country could see a wave of migrants fleeing Argentina if leftists were to return to power in presidential elections. Key Latin American stock indexes and currencies at 0813 GMT: Stock indexes Latest Daily % change MSCI Emerging Markets 973.49 -0.78 MSCI LatAm 2,660.51 -3.84 Brazil Bovespa 101,971.29 -2 Mexico IPC 39,842.01 -1.43 Chile IPSA 4,826.66 -1.76 Argentina MerVal 27,539.08 -30 Colombia IGBC 12,453.04 -0.52 Currencies Latest Daily % change Brazil real 3.9791 -1 Mexico peso 19.5824 -0.93 Chile peso 714 -0.41 Colombia peso 3,424.53 -0.92 Peru sol 3.381 -0.03 Argentina peso (interbank) 52.1500 -15.84 (Reporting by Susan Mathew in Bengaluru;)