CARACAS, Venezuela — A man crossing a street blocked by anti-government protesters was shot and killed yesterday in northern Venezuela, the government said, in a sign of rising tensions ahead of a referendum on constitutional change.

Vice President Jorge Rodriguez said protesters shot the 19-year-old man in Guacara, just outside the city of Valencia, during one of many demonstrations across the country before Sunday’s vote on constitutional amendments President Chavez says will help bring socialism to Venezuela. Mr. Rodriguez said at least 80 people were detained in the protests.

Protest organizers could not be immediately reached to comment on the official’s allegation.

“We must be alert. There are crazy and desperate parts of the opposition that started their plans for violence today in the cities of Maracay and Valencia,” Mr. Chavez said. “If they look for the path of violence, be sure that we will face them and sweep them aside.”

Mr. Chavez added that the authorities had established who the killer was, without elaborating.

If approved, the reforms would let Mr. Chavez run for re-election indefinitely, extend presidential terms from six to seven years, and create new types of property to be managed by cooperatives and communities.

Mr. Chavez’s opponents accuse him of concentrating power and seeking to be president-for-life like his close friend Fidel Castro of Cuba. Mr. Chavez insists he will only stay on as long as Venezuelans continue to vote for him.

University students have organized protests against the constitutional changes that have drawn tens of thousands and generated pro-government demonstrations in response, raising fears of violence.