A controversial migration deal with the United States hangs over the presidential election in Guatemala Sunday, where polls opened for voters to choose between former first lady Sandra Torres and opinion poll frontrunner Alejandro Giammattei.

Both candidates are vying for the ballots of more than eight million Guatemalans in the race to succeed corruption-tainted Jimmy Morales.

Electoral court president Julio Solorzano presided over the symbolic opening of the poll at 7:00 am (1300 GMT) from a public school in the historic center of Guatemala City.

Polling stations will close at 6:00 pm.

Solorzano urged citizens to avoid violence if the vote went against their preferred candidate.

"If the results don't favor X or Y political organization you have to accept it, and if you have issues then raise them," he said.

He said preliminary results would be published in real time on the electoral court's official website to provide transparency.

Corruption was the main issue leading up to the first round of elections in June -- which Torres topped -- but that has been superseded by the political scandal over the migration deal with the United States.

Neither candidate arrives with a glowing reputation, both having failed in previous bids for the presidency.

The center-left Torres, whose ex-husband Alvaro Colom was president from 2008-12, has been suspected of involvement in corruption before.

Influential businessman Dionisio Gutierrez recently described her as "a questionable politician with a history that should worry any citizen."

Giammattei, a conservative, has hardly come off any better. Investigative website Nomada branded him as "impulsive... despotic, tyrannical... capricious, vindictive," among other undesirable traits.

But the 63-year-old, a doctor by profession, scores well on voter concerns such as the economy, corruption and security, according to Risa Grais-Targow of the Eurasia Group.

Should he win, though, she said he "would face a lose-lose scenario" regarding the migration pact.

'Major Economic Strain' 

One of Morales's last acts as president was to authorize an agreement with the US administration of Donald Trump designating Guatemala as a "safe third country," meaning the US can turn away asylum seekers who have passed through the Central American country without seeking refuge there.

The pact -- part of Trump's campaign to stem migration at his country's southern border -- has proved highly unpopular in Guatemala, with demonstrators blocking roads and occupying the University of San Carlos.

In a poll by Prodatos for the Prensa Libre newspaper, 82 percent of respondents opposed it.

Grais-Targow believes the agreement would place "a major strain on the economy" if it overcomes legal challenges and takes effect.

The deal would allow the US to send most Honduran and Salvadoran asylum seekers who passed through Guatemala back to the poor, crime-stricken Central American country -- an influx it is ill-prepared to receive.

Rejecting the migration pact would run the "risk of retaliation from Trump," Grais-Targow said, after the US leader threatened a travel ban, tariffs and remittance fees if the country didn't bend to his will.

Remittances from Guatemalans in the US are a crucial part of the economy, reaching a record $9.3 billion last year. That compares to Guatemala's export revenue of $10.5 billion.

According to the World Bank, remittances account for 12 percent of the country's GDP.

Torres and Giammattei have avoided committing to strong positions over the migration pact.

The agreement was reached last month despite Guatemala's constitutional court having granted an injunction blocking Morales from signing the deal.

Entrenched Corruption 

Both candidates have concentrated their campaigns on attacking entrenched corruption and promising to improve education and health care, while investing in poorer areas to reduce poverty and discourage Guatemalans from seeking the "American dream."

Almost 60 percent of Guatemala's 17.7 million citizens live in poverty.

Combating gang violence is another major issue in a country with one of the highest murder rates in the world.

Around half the killings are blamed on drug trafficking and extortion operations carried out by powerful gangs.

Morales, barred by Guatemalan law from seeking a second term, leaves after four years in office with his popularity at rock bottom and the attorney general's office looking to investigate him for corruption.

Morales was elected on the promise of a clean government, but he tried to shut down the UN's International Commission Against Impunity in Guatemala (CICIG) when it started investigating him over illegal campaign financing.