MEDELLÍN, Colombia — President Nicolás Maduro of Venezuela ordered its border with Brazil closed Thursday, part of his escalating effort to stop opponents from sending humanitarian aid into his poverty-stricken country.

Juan Guaidó, the parliamentary opposition leader who declared himself president of Venezuela last month, has vowed to bring tons of aid donated by the United States and others into Venezuela on Saturday. Mr. Maduro, who has called Mr. Guaidó an American lackey, has vowed not to let the aid in.

The Brazil-border closure, announced on television, followed a Venezuelan government announcement on Wednesday that it would block air and sea travel between Venezuela and three Caribbean islands where the opposition said it wished to stage relief supplies.

“It’s better to prevent than to regret,” said Mr. Maduro.

The move comes 48 hours ahead of the deadline set by the opposition for Mr. Maduro to end his aid blockade or have it broken by force. The situation has led to a tense standoff between the two sides and to fears of violence.