By Sofia Menchu and Diego Oré

GUATEMALA CITY (Reuters) -Former first lady Sandra Torres is set to face an August run-off in Guatemala’s presidential election against another center-left candidate running on an anti-graft ticket who bucked forecasts to finish runner-up in a first round of voting on Sunday.

Preliminary results showed that with more than 98% of returns from polling stations counted, Torres of the National Unity of Hope (UNE) had 15.8% of the vote, and Bernardo Arevalo of Semilla, another left-of-center party, 11.8%.

The election, which has been dominated by concern over corruption in the Central American country, is set for a decisive second round on Aug. 20 because no candidate came close to winning the 50% plus one vote needed for outright victory.

Torres told a press conference on Sunday night she was feeling optimistic. “We’re happy,” she said. “We’re going to win, against whoever it may be.”

Arevalo, an ex-diplomat and son of former president Juan Jose Arevalo, was the surprise package of the vote, with opinion polls not forecasting he would make the run-off. Arevalo has made tackling corruption a key priority.

“We didn’t come to win the polls. We came to win the elections,” Arevalo wrote on Twitter.

His party’s previous presidential campaign was fronted by former attorney general and anti-corruption stalwart Thelma Aldana, though she was ultimately barred from running in 2019 on the grounds of alleged financial impropriety.

Aldana said the allegations were politically motivated due to the historic campaign against graft she waged together with the U.N.-backed International Commission Against Impunity in Guatemala (CICIG).

Aldana later sought asylum in the United States.

Guatemala’s Supreme Electoral Tribunal is expected to announce definitive results later on Monday after an election in which there was widespread clamor for the next leader to tackle poverty, corruption and violence in the country.

In the end, the process frustrated many voters.

Nearly one in four ballots were spoiled or left blank, as Guatemalans expressed discontent at the decision to bar early front-runner, businessman Carlos Pineda. Pineda urged supporters to spoil their ballots after he was ruled ineligible.

Running third in the race on 7.8% of the vote was Manuel Conde, candidate of conservative President Alejandro Giammattei’s ruling Vamos party, results showed.

Preliminary results pointed to a fragmented Congress which could make it hard for the next president to govern.

Polls have suggested that the 67-year-old Torres will struggle to win a run-off given her unpopularity in the capital, Guatemala City, home to a high percentage of the electorate. She finished runner-up in the last two presidential elections.

The ex-wife of Alvaro Colom, Guatemala’s president from 2008 to 2012, Torres was competing with over 20 other candidates, including Edmond Mulet, a career diplomat, and Zury Rios, daughter of the late right-wing dictator Efrain Rios Montt.

The contest to succeed the outgoing Giammattei, who is limited by law to a single term, was overshadowed by a court ruling that blocked four candidates, including Pineda.

The United States and the European Union criticized the exclusion of Pineda, who called the decision “electoral fraud.”

Meanwhile, unrest on Sunday in the town of San Jose del Golfo, near the capital, forced the postponement of voting there to August, the Supreme Electoral Tribunal said.

(Writing by Dave Graham and Natalia Siniawski; Editing by Stephen Eisenhammer, Toby Chopra and Alistair Bell)

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