MARGARET WARNER:

The peace was broken almost immediately, as protesters clashed with police and government supporters. National Guard troops fired tear gas, and a teenage boy died after being shot in the head.

Demonstrators have tried repeatedly in recent days to reach the capital city's downtown. Each time, they were turned back by riot police. At least five people were killed, and hundreds arrested.

Brian Ellsworth of Reuters, speaking to us by Skype from Caracas, said tensions have been building.