The Iraqi government faced more pressure late Saturday from a surprise sit-in by lawmakers linked to populist cleric Moqtada al-Sadr amid persistent mass protests across the country.

Thousands of Iraqis returned to the streets in the capital Baghdad and across the south over the last two days, defying curfews and violence that have left over 60 dead.

Sadr, a firebrand former militia leader, has thrown his weight behind the protests by demanding the government resign and Iraq hold early parliamentary elections.

Saturday night, members of his Saeroon bloc - parliament's largest with 54 MPs - escalated the situation.

MP Badr al-Zayadi said they had begun an open-ended "sit-in, until the enactment of all reforms the Iraqi people are demanding."

Zayadi said that the bloc had sent an "official request" to Iraq's president, who according to the constitution can ask parliament to withdraw confidence from the premier.

The move has put Prime Minister Adel Abdel Mahdi under more pressure than ever as Saeroon was one of the two main sponsors of his government.

He faced relentless protests on Friday despite efforts in Baghdad and across the south to clamp down on them.

Late Saturday, street lights were turned off around Baghdad's Tahrir (Liberation) square, plunging protesters clinging onto their positions there into darkness.

Witnesses said security forces were unleashing volleys of tear gas to clear the crowds, each canister lighting up the smoke-filled square with flashes of light.

Explosions could be heard from the square well past midnight.

The latest round of demonstrations has been notably violent, with 63 people killed and more than 2,000 wounded over just two days, according to the Iraqi Human Rights Commission.

That has pushed the death toll for so far this month to 220.

The United Nations said it was "tragic" to see renewed violence but also warned against "armed spoilers".

"Armed entities sabotaging the peaceful demonstrations, eroding the government's credibility and ability to act, cannot be tolerated," said the UN top official in Iraq, Jeanine Hennis-Plasschaert.

(With input from AFP)