Police have turned back thousands of protesters who tried to reach the headquarters of Venezuela's electoral body.

Demonstrators tried to march to the building in downtown Caracas Tuesday to demand the government allow a recall referendum against President Nicolas Maduro.

Long lines of police in riot gear blocked their way. One group led by opposition leader Henrique Capriles tried to advance anyway, and was turned back with tear gas.

Tuesday's protest is at least the fourth in as many weeks. Each time, police have stopped protesters from reaching downtown.

The government begun a tentative dialogue process last month with the opposition coalition mediated by former world leaders. On Tuesday, the Union of South American Nations said the opposition had not shown up to the most recent meeting.