(Updates with quote from ambassador, context)

GUATEMALA CITY, Sept 18 (Reuters) - The United States urges Guatemalan authorities to end their "intimidation efforts" targeting election officials and members of the party voted to power in last month's presidential elections, the U.S. Ambassador to the Organization of American States said on Monday.

Speaking to the OAS permanent council, Ambassador Francisco Mora said the U.S. was concerned about efforts to undermine democracy in Guatemala, including the prosecutor's office recently raiding electoral storage facilities and opening sealed ballots.

"In a healthy democracy, institutions don't tamper with ballot boxes after election results have been officially certified by the appropriate authority," Mora said, adding the act represented "an assault on the rule of law."

Last week, the top prosecutor's office in the Central American country

raided facilities

run by Guatemala's main electoral tribunal as part of an investigation into the lead-up to the elections which saw anti-graft candidate Bernardo Arevalo and his Semilla Party overwhelmingly come out on top.

Arevalo suspended his participation in the transition of power last week until "necessary institutional (and) political conditions are reestablished."

He said on Monday he would analyze the possibility of restarting the transition process, speaking to press as he presented a legal challenge against the investigation to Guatemala's Supreme Court.

Hundreds of protesters gathered in front of the court to accompany Arevalo, waving flags and holding signs that read "out with the coup-mongers."

More protests are expected to continue on Tuesday, with roadblocks planned throughout the country in support of Arevalo and to demand the resignation of the prosecutor investigating him.

(Reporting by Sofia Menchu and Diego Ore; Additional reporting by Kylie Madry; Editing by Sarah Morland)