MONTEVIDEO (Reuters) - Uruguay’s conservative opposition has posted a big win in presidential primaries, firing a warning at the ruling coalition ahead of national elections in October as the South American nation grapples to revive economic growth.

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The official vote count, announced early on Monday, showed former Montevideo mayor Daniel Martínez and Senator Luis Lacalle Pou winning the nominations for the ruling Broad Front coalition and the National Party respectively, the two main factions expected to battle it out later this year.

Uruguay’s electoral system allows voters to select both a preferred presidential candidate for each of the three main parties and a preferred party. Sunday’s turnout was around 40%.

Uruguay’s electoral court said that, with 99% of the vote counted, the Broad Front had received 23.6% of the overall vote, while the National Party gained 41.6%. The third main political faction, the Colorado Party, gained 16.8%.

The presidential ballot will be a test for the center-left ruling party, led currently by Tabaré Vázquez, which is grappling with sluggish economic growth, weighed down by the effects of drought and flooding on the crucial farm sector.

Martinez, a 62-year-old engineer, beat out Carolina Cosse, former minister of industry whose candidacy was backed by ex-President José Mujica. Also competing were former Central Bank president Mario Bergara and trade unionist Óscar Andrade.

Lacalle Pou, 44, a lawyer and the son of former president Luis Alberto Lacalle, was facing Senator Jorge Larrañaga and businessman Juan Sartori, who joined the party at the end of last year and had been surging lately in the polls.

The National Party was last in power in 1990, while Broad Front, a coalition of left-wing parties, got into power by winning the presidential election in 2004.

“Uruguay has started to decide it wants a change of government because it’s starting looking away from the Broad Front and looking for someone to trust,” Lacalle Pou said.

Martinez said the country needed renovation, chance and unity “facing a more-than-challenging world”.

Ernesto Talvi, the 62-year-old economist who will be the Colorado Party candidate, is a relative newcomer, though he beat out more experienced rival Julio María Sanguinetti, an 83-year-old lawyer who has twice been president.

Polling data ahead of the Oct. 27 election have previously suggested a close race between the Broad Front party and the National Party, with a likely run-off in November, which happens if there is no clear winner in the first round.