CÚCUTA, Colombia — An ambitious plan by Venezuela’s opposition to peacefully import foreign aid in truck convoys degenerated into deadly skirmishes Saturday along the impoverished country’s borders, with a smattering of supplies getting through but most of it blocked by armed loyalists of President Nicolás Maduro.

The aid deliveries, promised by the opposition leader, Juan Guaidó, have been viewed as a major test of his credibility in the month since he declared himself president and promised to call for new elections and end Venezuela’s economic free-fall.

“The humanitarian aid is definitely on its way to Venezuela, in a peaceful manner, to save lives right now,” said Mr. Guaidó, standing in front of a warehouse on the Colombian side of the border.