Police fired tear gas and rubber bullets to break up thousands of marchers as they reached a vital freeway in Caracas, then edged back slightly as masked protesters pelted them with stones and Molotov cocktails.

Demonstrators vowed not to flinch in their campaign to oust Maduro, despite three weeks of protest violence that has left eight people dead, including three on Wednesday.

"Sure we're tired, but we've got to stand strong. I'm ready to take to the streets every day if I have to," said 22-year-old student Aquiles Aldazoro.

Protesters set fire to trash bins and tore down a billboard to barricade themselves in, as officers fired water cannon at them and a police helicopter hovered overhead.

"I don't care if I inhale gas, I don't care if I die. We have to put a stop to this murdering, repressing government," said Natasha Borges, 17.

One protester, completely naked except for sneakers, walked up to police and shouted: "Please, stop gas-bombing us."

Another draped in a Venezuelan flag defiantly marched up to an armored truck and stared it down.

Clashes also broke out in the flashpoint western city of San Cristobal, Maracaibo in the northwest and Valencia in central Venezuela.

The opposition called for new protests, a "march of silence" in all parts of the country on Saturday and a national road blockage on Monday.