Venezuelan authorities confirmed a young man was killed during anti-government protests and vowed to investigate the death, the first since a controversy over the Supreme Court blew up last week.

The march of thousands of people culminated in clashes on Thursday and opposition leaders said Jairo Ortiz was shot in a Caracas suburb by security forces who were breaking up a protest there.

"In the face of the vile assassination of the young Jairo Ortiz, we manifest our firm condemnation of such a vile act," state human rights ombudsman Tarek Saab said on Twitter early on Friday.

Venezuela has seen near-daily protests since the Supreme Court issued a ruling nullifying Congress last week, a move the opposition called a "coup d'etat".

While the widely condemned court decision was quickly overturned, the opposition has stepped up street protests.

On Friday, demonstrators against socialist President Nicolas Maduro gathered at the ombudsman's office from dawn, wrapping red tape emblazoned with the words "Danger, do not enter" around it in a surprise protest.

"We declare this office closed because its back is to the people," tweeted opposition legislator Milagros Paz. "Jairo Ortiz died in the hands of this regime."

Maduro said in a televised address on Thursday that authorities had detained 30 people involved in the demonstration as the country's fragmented opposition gained new impetus against a government it blames for the country's social and economic meltdown.

The South American country is suffering from triple-digit inflation, shortages of basic foods and medicines, and one of the world's highest murder rates.

Maduro's government has said a US-backed business elite is responsible for Venezuela's economic downturn, and it is trying to foment a coup to impose right-wing rule.