Venezuelan authorities said Sunday they arrested six suspects tied to an alleged plan to assassinate President Nicolás Maduro using a pair of drones armed with explosives, showing the leader’s fragile hold on power amid a crippling economic crisis.

Critics warned that Mr. Maduro could use the purported plot, which injured seven soldiers and interrupted him as he addressed a military parade Saturday in downtown Caracas, as a pretext to intensify a crackdown on dissidents.

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