Iraqs rival parties were negotiating the contours of a new government on Monday, after the previous cabinet was brought down by a two-month protest movement insisting on even more deep-rooted change. After just over a year in power, premier Adel Abdel Mahdi formally resigned Sunday after a dramatic intervention by top Shiite cleric Grand Ayatollah Ali Sistani. That followed a wave of violence that pushed the protest toll to over 420 dead -- the vast majority of them demonstrators. The United States on Monday called recent violence in Nassiriya, Iraq in which at least 29 people died “shocking and abhorrent,” calling on the Iraqi government to investigate and punish those responsible for the “excessive” use of force. Iraqi security forces opened fire on demonstrators who had blocked a bridge and later gathered outside a police station in the southern city, killing at least 29 people. Police and medical sources said dozens more were wounded. Iraqi forces have killed over 400 people, mostly young, unarmed protesters, since mass anti-government protests broke out on October 1. More than a dozen members of the security forces have also died in clashes. “The use of excessive force over the weekend in Nassiriya was shocking and abhorrent,” David Schenker, US assistant secretary of state for Near Eastern affairs, told reporters. “We call on the Government of Iraq to respect the rights of the Iraqi people and urge the government to investigate and hold accountable those who attempt to brutally silence peaceful protesters,” he added. The unrest is Iraq’s biggest challenge since ISIS seized swathes of Iraqi and Syrian territory in 2014. It pits mostly young, disaffected Shiite protesters against a Shiite-dominated government that is backed by Iran and has been accused of squandering Iraq’s oil wealth while infrastructure and living standards deteriorate. Parliament on Sunday formally tasked the president with naming a new candidate, but Iraqs competing factions typically engage in drawn-out discussions before any official decisions are made. Talks on a new premier began before Abdel Mahdi resigned, a senior political source and a government official told AFP. "The meetings are ongoing now," the political source added. Such discussions produced Abdel Mahdi as a candidate in 2018, but consensus will be harder this time around. "They understand it has to be a figure who is widely accepted by the diverse centers of power, not objected to by the marjaiyah (Shiite religious establishment), and not hated by the street," said Harith Hasan, a fellow at the Carnegie Middle East Center. The candidate would also have to be acceptable to Iraqs two main allies, arch-rivals Washington and Tehran. "The Iranians invested a lot in the political equation in the last few years and wont be willing to give up easily," said Hasan, according to AFP. Tehrans pointman on Iraq, Qassem Soleimani, who heads the Iranian Revolutionary Guard Corps foreign operations arm, arrived in Iraq last week for talks, official sources said.Two sides of the same coin The United States said that Soleimanis presence showed that Iran was again "interfering" in Iraq, accusing Tehran of having "exploited" the neighboring country. Despite the oil wealth of OPECs second-biggest crude producer, one in five Iraqis lives in poverty and youth unemployment stands at one quarter, the World Bank says. Demonstrators say such systemic problems require more deep-rooted solutions than Abdel Mahdis resignation. "We demand the entire government be changed from its roots up," Mohammad al-Mashhadani, a doctor protesting in Baghdads Tahrir Square, said on Monday. Nearby, law student Abdelmajid al-Jumaili said that meant parliament and even the president would have to go. "If they get rid of Abdel Mahdi and bring someone else from the political class, then nothing changed. Theyd just be two sides of the same coin," said Jumaili. But the protesters demand for an entirely new face has complicated the search for a new premier. Two political heavyweights said they opted out of current talks: former premier Haidar al-Abadi and cleric Moqtada Sadr, who had backed the government until protests erupted. "Theyre aware the bar is too high and its too difficult for them to please the street," said Hasan. At the same time, a totally new player is unlikely to be trusted by the established political class. "The discussions now are over someone from the second or third tier of politicians," the government source told AFP. "Its not possible to have someone new. It has to be someone who understands the political machine to push things along."Many firsts for Iraq The government and political sources said parties were considering a "transitional" cabinet to oversee electoral reform before an early parliamentary vote. "This process will take no less than six months," the official said. A new voting law has been a key demand of protesters as well as Sistani, and is now a centerpiece of the governments proposed reforms. On Monday, Parliament Speaker Mohammed al-Halbusi held meetings with the main legislative blocs, the United Nations and parliaments legal committee to discuss a draft law, with more talks expected on Tuesday. President Barham Salih met the UNs top representative in Iraq, Jeanine Hennis-Plasschaert, as well as the European Unions ambassador to the country. Iraqs entrenched political elite has been struggling to "think outside the box" to resolve the crisis, Hasan said. Beyond the rising death toll from protests, rights groups have slammed the harassment, kidnapping and even killing of activists, medics and regular protesters in recent months. "Authorities are failing Iraqi citizens by allowing armed groups to abduct people, and it will be up to the government to take swift action against these abuses," said Human Rights Watchs regional director Sarah Leah Whitson.