Argentine President Mauricio Macri vowed on Monday to win a second term after a surprisingly strong performance by the opposition in the primary election sent shocks through markets, crashing the peso and sending stocks and bonds tumbling. Mr Macri said he would reverse the result of Sunday's primary but admitted that a weaker peso triggered by the surge in support for opposition candidate Alberto Fernandez and his running mate, former president Cristina Fernandez, would fuel inflation. The peso closed 15 per cent weaker after dropping about 30 per cent to a record low earlier in the day. The primary election prompted market fears of Argentina's possible return to the interventionist economics of the previous government. Refinitiv data showed Argentine stocks, bonds and the peso had not recorded this kind of simultaneous fall since the South American country's 2001 economic crisis and debt default. "I trust we will have a more even election in October that will allow us to go to a second round," Mr Macri said. "Those who don't want to go back to the past will join us." He said he was not considering a Cabinet reshuffle. Mr Fernandez, a former Cabinet chief, dominated the primary vote by an unexpected 15.5 percentage points over the president. He has said he would seek to "rework" Argentina's $57 billion (Dh209bn) standby agreement with the International Monetary Fund if he won October's general election. Despite Mr Macri's struggles to turn the economy around, investors regard the Fernandez ticket as a riskier prospect. Argentine stocks were among the top losers on the Nasdaq index, and the local Merval stock index closed 31 per cent weaker. Falls of between 18 and 20 cents in Argentina's benchmark 10-year bonds and century bond left them trading at about 60 cents on the dollar or even lower. Morgan Stanley downgraded its recommendation for Argentina's sovereign credit and equities from "neutral" to "underweight", and said calculations suggest the peso could fall another 20 per cent. "Given the narrow path ahead, our view is that a high degree of policy continuity is a must in order to maintain macro stability," Morgan Stanley said in a research note. It predicted "further downside pressure and volatility" in bond prices. Argentina's Central Bank intervened, selling $105 million in the foreign exchange market to defend the peso in the face of the massive sell-off. The auction used the bank's own reserves for the first time since September of last year, traders said.