By Sofia Menchu

GUATEMALA CITY (Reuters) -A judge in Guatemala suspended the party of anti-graft presidential candidate Bernardo Arevalo, a prosecutor at the country's Attorney General's Office said on Wednesday, apparently removing him from the race.

In a video posted on Twitter, Prosecutor Rafael Curruchiche said investigations showed irregularities in the registration of more than 5,000 members of the Semilla party.

Shortly after the announcement, Guatemala's electoral court formally confirmed the first round results, which put Arevalo in the second round and said they had not been notified of the suspension.

"It is something that concerns us as a court, because we know that elections are won at the polls," Irma Palencia, head of the electoral court, said when asked about the suspension.

Results from the election's first-round vote on June 25 showed Arevalo facing former first lady Sandra Torres in the country's presidential run-off set for next month.

Formalizing the initial results was delayed until Wednesday after the ruling party and allies alleged irregularities in the vote which triggered a review of the ballots.

Polls had put Arevalo, an ex-diplomat and son of former president Juan Jose Arevalo, as a distant outsider but his surprising second place finish led many analysts to believe he could win the whole thing.

"Given the unusual but predictable news... We are analyzing the case with our legal team," Semilla's spokesperson said in a message sent to media.

The electoral process in the Central American country has been strongly criticized by international and national organizations due to the delay in official results and the suspension of other presidential hopefuls earlier in the process.

A judge suspended the candidacy of early frontrunner Carlos Pineda in May, in a ruling the businessman decried as undemocratic.

Will Freeman, a fellow for Latin America studies at the Council on Foreign Relations, said the decision against Semilla was deeply concerning.

"If this decision stands, Guatemala is headed toward becoming the new Nicaragua," he said. "Everyone in the political class expected Semilla would win a free and fair vote and this proves it."

Critics have slammed outgoing conservative President Alejandro Giammattei for undermining anti-corruption efforts, which has led many judges and prosecutors to either flee or risk arrest. Giammattei is barred by law from a second term.

(Reporting by Sofia Menchu; Editing by Brendan O'Boyle, Stephen Eisenhammer and Lincoln Feast.)