GUATEMALA CITY — The president of Guatemala pushed the nation toward a constitutional crisis on Monday, ignoring an explicit order by the country’s top court while testing the bounds of the nation’s fragile democracy.

The court had ordered that the head of a United Nations-backed anti-corruption commission be allowed back into Guatemala, reversing orders from President Jimmy Morales.

But the government stood its ground on Monday, saying it would not allow the head of the commission, Iván Velásquez, a Colombian citizen, to return. Instead the government said the United Nations, which appointed Mr. Velásquez to oversee the International Commission Against Impunity in Guatemala, should pick someone else to head the mission.

The announcement not only defied the constitutional court’s unanimous ruling late Sunday, but it was also a challenge to United Nations Secretary General António Guterres. In a rebuke to the Guatemalan government, Mr. Guterres had said at the beginning of the month that Mr. Velásquez would remain at the head of the panel even though he was barred from the country.