CÚCUTA, Colombia—Venezuelan soldiers fired on protesters, and the country’s U.S.-backed opposition leader defied President Nicolás Maduro by crossing into Colombia, raising tensions ahead of an expected showdown Saturday over the delivery of tons of humanitarian aid.

International help has emerged as a flashpoint in Venezuela’s political and economic crisis. Mr. Maduro is halting deliveries, which he calls a U.S.-backed coup attempt.

The...