By Kylie Madry and Cassandra Garrison

International leaders celebrated Bernardo Arevalo's resounding victory in Sunday's Guatemalan presidential election, a win which long seemed out of reach for the anti-graft candidate in a process dogged by accusations of government meddling.

Arevalo, a 64-year-old former diplomat and son of Guatemala's first democratically elected president, captured 58% of votes versus former first lady Sandra Torres' 37%, with nearly all votes counted late Sunday.

European Union foreign policy chief Josep Borrell said the election result was "very clear" and hailed the Guatemalan government's pledge to ensure an orderly handover of power.

"It is crucial for all state institutions and all sectors of society to support and join in this effort in the interests of the country," Borrell said in a statement.

His congratulations were echoed by neighboring leaders including Mexican President Andres Manuel Lopez Obrador and President Xiomara Castro of Honduras.

Luis Almagro, head of the Organization of American States (OAS), said in a post on X, formerly known as Twitter, that Guatemala had had an "exemplary election day."

Arevalo, who ran an anti-corruption campaign, had faced an uphill battle. Defying polls, he claimed a surprise second place in a first-round vote in June, triggering a runoff. A number of other opposition candidates had been barred from running.

His competitor Torres alleged irregularities in the first round of voting and Arevalo's party, Semilla, was briefly suspended at the request of a top Guatemalan prosecutor.

By Monday morning, Torres had yet to accept her loss publicly. In a press conference Sunday afternoon, she said she was "worried" about the integrity of the vote.

Her UNE party said it would take a position once the elections results were put out "with total transparency."

An OAS representative, with a team of 86 observers in Guatemala, said voting went smoothly and the election "fulfilled all the demanding obligations." An EU mission is expected to put out a preliminary statement about its findings on Tuesday.

TRANSITION WORRIES

The EU, as well as governments such as Brazil and Norway, said they expected a peaceful transition of power, which outgoing President Alejandro Giammattei vowed to guarantee.

However, attempts to stymie Arevalo are likely, said Risa Grais-Targow, an analyst at consultancy Eurasia Group.

"The ruling pact will likely continue to target electoral officials and Arevalo's Semilla party with investigations ahead of January's change in government," she said.

Giammattei said on X he had congratulated Arevalo, and invited him to meet the day after results were finalized.

Lacking a majority in Congress, Arevalo faces a tough transition and challenges in office. Guatemala has been roiled by violence and food insecurity and its citizens make up the largest number of Central Americans trying to enter the U.S.

Still, his win was "an opportunity to rebuild the rule of law in a country where those in power have used their position to protect themselves and their cronies," said Tamara Taraciuk of U.S.-based think tank the Inter-American Dialogue.

From a macroeconomic standpoint, Arevalo will take charge of a country with strong fundamentals, but will likely struggle to implement his social agenda, analysts at JPMorgan wrote.

He will also need to get momentum going to reverse the damage done to Guatemala's democracy, said former U.S. ambassador to the country Donald Planty.

"That's not something one reverses in a short period of time," Planty said, but added he hoped Arevalo can leave progress for his successor to build on.

TAIWAN

After polls closed, Arevalo said he had already spoken with Mexican leader Lopez Obrador and Salvadoran President Nayib Bukele about Guatemala's agenda with its neighbors.

Taiwan also said it congratulated Arevalo on the win, despite the candidate's promise to strengthen Guatemala's ties with China alongside its longstanding alliance with Taiwan.

Guatemala is one of only 13 countries to maintain formal diplomatic ties with the Chinese-claimed island, and China holds that no country it has ties with can maintain separate diplomatic relations with Taipei.

(Reporting by Kylie Madry and Cassandra Garrison; Additional reporting by Gabriel Araujo; Editing by Chizu Nomiyama, Jonathan Oatis and David Gregorio)