In capital Baghdad and southern cities, demonstrators chant "we reject Allawi" and hold posters of the new PM's face with a red cross through it a day after Mohammed Tawfiq Allawi's appointment.
Furious anti-government youth dug in their heels in Iraq's capital and south on Sunday, rejecting the previous evening's nomination of Mohammed Tawfiq Allawi as premier after months of demonstrations and political paralysis.
Young demonstrators have expressed contempt for the ruling elite and on Sunday, they slammed Allawi – a former lawmaker and minister – as part and parcel of the system they want to overhaul.
"Mohammad Allawi is rejected by order of the people," read a new sign hung in the holy city of Najaf on Sunday.
Young men with their faces wrapped in checkered scarves had spent the night torching car tyres in anger at Allawi's nomination and smouldering remains still blocked the main roads on Sunday, according to a reporter on the ground.
In Diwaniyah, further south, protesters marched into government buildings to demand they close for the day while students began sit-ins at schools and universities.
Protesters in Hillah blocked off all roads leading into the city and chanted, "Allawi is not the people's choice".
Allawi was named prime minister-designate after a hard-won consensus among Iraq's rival parties, who had struggled to agree on a candidate since outgoing premier Adil Abdul Mahdi resigned under growing street pressure two months ago.
Mass rallies have rocked Baghdad and the mainly-Shia south since October, with protesters demanding snap elections and an independent prime minister, as well as accountability for corruption and recent bloodshed.
Students revolt in Baghdad
In Baghdad, hundreds of students flooded the streets around the main protest camp of Tahrir Square, carrying Allawi's photograph with an "X" over his face.
"We are here to reject the new prime minister because he has a well-known history within the political class," said 22-year-old university student Tiba.
Allawi, 65, served as a parliamentarian immediately after the 2003 US-led invasion that toppled former ruler Saddam Hussein, then was twice appointed communications minister under former prime minister Nuri al Maliki.
But he resigned both times, accusing Maliki of turning a blind eye to graft in a country considered among the top 20 most corrupt in the world by Transparency International.
His appointment came after days of crisis talks prompted by President Barham Saleh, who said he would select his own candidate if the political blocs of Iraq's parliament did not nominate someone by Saturday.
Allawi had been among the top contenders for the post, but the stalemate continued into the late afternoon, Iraqi government sources told AFP news agency on condition of anonymity.
The talks were very secretive and it remains unclear what finally unlocked a deal, but on Saturday evening, Allawi announced his own nomination in a video posted to Twitter.
There was no official statement from Saleh, but Abdil Mahdi congratulated his successor and the pair pledged to meet soon to ensure a smooth transition.
Sadr seeks to curb rallies
In his first formal address, Allawi vowed to form a representative government, hold early parliamentary elections and ensure justice for protest-related violence.
More than 480 people have died and nearly 30,000 have been wounded since the rallies began on October 1, but few have been held accountable for the bloodshed.
Allawi has one month to form his government, but ensuring its independence may prove a challenge, said Sajad Jiyad of the Iraq-based think tank the Bayan Center.
"If we've learned anything from the previous PM, it's that this is the most difficult part: pushing back against the political blocs' demands," Jiyad said.
In Iraq, cabinets are typically formed after complex horsetrading whereby parties demand lucrative and influential ministerial posts based on their share of parliament.
If Allawi fails to resist ministerial candidates proposed by parties, "it will back up what protesters are saying" about his allegiance to the factions, Jiyad added.
Top cleric backs new PM
Among the most powerful of Iraq's political players is populist cleric Moqtada Sadr, who welcomed Allawi's nomination on Saturday as a "good step".
Sadr backed the protests in October and his die-hard followers are widely recognised as the most organised and well-equipped among anti-government demonstrators.
But on Sunday, Sadr ordered members of his movement to organise with security forces to reopen roads and schools, contrary to what other protesters across Iraq were doing.
"The revolution must go back to being restrained and peaceful," Sadr wrote on Twitter.
That tweet and his endorsement of Allawi were seen as a betrayal by other protesters in Baghdad, who chanted "don't tweet however you please," hinting at Sadr, and "we are a young, leaderless revolution".
Iran welcomes election of Allawi
Iran also welcomed Allawi's election as prime minister of Iraq, Iran's foreign ministry spokesman Abbas Mousavi said on Sunday, according to the official IRNA news agency.
"In continuing support for the independence, nationals overeignty, territorial integrity and strengthening the foundations of democracy in Iraq, Iran welcomes the election of Mohammed Tawfiq Allawi as the new prime minister of this country," Mousavi said.