Argentina's opposition candidate Macri takes surprise lead in election

BUENOS AIRES (Reuters) - Conservative opposition candidate Mauricio Macri took a surprise lead in Argentina's presidential election on Sunday, almost certainly forcing the ruling party's Daniel Scioli into a run-off vote next month, partial results showed.

Scioli, backed by outgoing leftist president Cristina Fernandez, had hoped for an outright victory on Sunday.

But with 71 percent of ballots counted, preliminary results showed Macri, the business-friendly mayor of Buenos Aires, leading with 36 percent of the vote while Scioli had 35 percent.

"What happened today will change politics in this country," Macri said in a speech to supporters, rallying them ahead of an expected Nov. 22 second round of voting.

Scioli could still overtake Macri as the remaining votes come in but the two men are now virtually certain to go to a run-off election next month and Scioli looks vulnerable.

In a speech before thousands of party militants, Scioli earlier reached out to swing voters for their support without explicitly accepting the vote would go to a second round.

"United together we will triumph," Scioli told voters in a rallying call. "I call upon the undecided and independent voters to join this cause."

(Additional reporting by Hugh Bronstein, Maximiliano Rizzi, Juliana Castilla and Maximiliano Rizzi; Writing by Richard Lough; Editing by Kieran Murray and Christian Plumb)