For more than three months, the Venezuela opposition leader, Juan Guaidó, has been exhorting the armed forces to join his side in ousting President Nicolás Maduro. On Tuesday Mr. Guaidó made his plea at a military base in the heart of Caracas, the capital, raising speculation that the military might be ready to heed him.

Clashes erupted outside the base and elsewhere as Mr. Maduro’s government and his supporters called Mr. Guaidó’s maneuver an attempted coup.