Guatemala's top court on Saturday ordered ballots from the first-round presidential election to be reviewed after the front-runner's party and allies challenged the results, setting up the potential for a recount ahead of the runoff vote.

Former first lady Sandra Torres is set to face off in the decisive second round on Aug. 20 with anti-graft candidate Bernardo Arevalo, who bucked forecasts to become the runner-up as anger over years of corruption scandals dominated voter sentiment.

Torres took 15.8 percent of the vote in the June 25 round, with Arevalo close behind with 11.8 percent. Some analysts expect Arevalo to win the runoff due to Torres' unpopularity in the voter-dense capital, Guatemala City.

The court said it would suspend election processes in the Central American country until the ballots are reviewed, acting at the request of Torres' National Unity of Hope party, President Alejandro Giammattei's Vamos party and several allied groups.

It said Guatemalan electoral authorities would check the votes, especially any thought to have irregularities, to make sure they met legal requirements.

The court said it would then determine whether to take the rare step of mandating a recount.

Arevalo said he would ask Guatemala's electoral court to annul the top court's decision, which he described as lacking legal merit and dangerous to the electoral process.

After his surprise success, Arevalo has called on Guatemalan voters to be on guard against attempts by his opponents to manipulate the outcome.

"We can't let the same old parties, frustrated and disappointed by their poor results in the first round, tarnish and call into question the free decision of thousands of Guatemalans," he said in a video on social media.

In the days after the vote, Torres said she was concerned that votes had been manipulated with a faulty software system to favor Arevalo's party, Semilla.

The Organization of American States, which observed the vote, said the process appeared to be carried out fairly, with few irregularities that did not alter the results.