“If these other countries don’t recognize Venezuela as a democracy, it will be hard for them to look like a legitimate power,” said David Smilde, an analyst at the Washington Office on Latin America, a human rights advocacy group. Mr. Smilde mentioned a list of consequences that such isolation could entail, from access to bank loans to straining diplomatic ties with its largest neighbors.

One candidate for the constituent assembly, José Félix Pineda, a 39-year-old lawyer, was killed in his home the night before the vote. Prosecutors said an armed group had broken into Mr. Pineda’s home in Ciudad Bolívar on Saturday night and shot him dead there.

Hours later, a large explosion rocked a middle-class neighborhood in the Venezuelan capital, Caracas, injuring at least seven police officers on patrol. Video circulating on social media showed the uniformed officers, all on motorcycles, riding into a fireball that had just erupted in front of them.

Nearby residents applauded as the security forces threw tear gas at them.

As the day wore on, government security forces used water cannons, rubber bullets and batons against protesters, just as they have for the last three months.