By Sofia Menchu

GUATEMALA CITY (Reuters) -Guatemalan presidential candidate Bernardo Arevalo is seen winning an Aug. 20 runoff election with 63% of valid votes against his rival, Sandra Torres, according to a CID Gallup poll published on Wednesday by a local think tank.

Arevalo won a surprise second place in the initial round in June, finishing close behind Torres, a former first lady.

Guatemalan authorities halted processing of the results and ordered the suspension of his anti-corruption party, Semilla, over the alleged illegal registration of party members, but the suspension was reversed by the country's top court last month.

The electoral process has raised international concern and the head of Organization of American States, which has a mission monitoring the elections, has flagged 'clear interference' with the vote.

The government has meanwhile said it is committed to ensuring peaceful elections.

CID Gallup's poll found that 54% of respondents considered Torres "corrupt" and a "liar," compared to 15% and 13%, respectively, for Arevalo. Arevalo led the poll in every age group, region, and education level.

"We didn't come to win in the polls, but to defeat the old guard in the ballot box," the candidate said on messaging platform X, formerly known as Twitter. "We must not be overconfident or rest on our laurels."

The study, presented by the Liberty and Development Foundation, surveyed 1,242 adults face-to-face between July 18 and 27 with an estimated margin of error of plus or minus 2%.

The poll comes a day after Guatemalan businessman and Duolingo Chief Executive Luis Von Ahn said he had contributed $100,000 to Semilla's campaign, in an usually public political declaration for the Guatemalan business sector.

(Reporting by Sofia Menchu and Raul Cortes; Writing by Sarah Morland; Editing by Stephen Eisenhammer and Conor Humphries)