Iraq’s political process was unraveling faster than had been anticipated Saturday, with Sunni politicians walking out of the nation’s parliament and threatening to resign from the government even before the last U.S. troops had left the country.

The crisis was triggered by reports that security forces loyal to Prime Minister Nouri al-Maliki, a Shiite, are planning to arrest the country’s Sunni vice president, Tariq al-Hashimi, and charge him with terrorism.

Those reports have fueled fears among Sunni politicians that Maliki intends to further consolidate his grip on power by moving against his rivals now that U.S. troops have gone. In recent days, the homes of top Sunni politicians in the fortified Green Zone have been ringed by tanks and armored personnel carriers, and rumors are flying that arrest warrants will be issued for other Sunni leaders.

The mostly Sunni Iraqiya bloc said it had withdrawn from parliament to protest what it called Maliki’s increasingly dictatorial behavior. Sunni ministers in the coalition government will resign unless he gives them a greater say in running the government and, in particular, overseeing the country’s Shiite-dominated security forces, the bloc warned.

Maliki loyalists accused the Sunnis of trying to forestall the detention of Hashimi, who, they say, has been definitively tied to acts of terrorism.

“His office is in charge of the funding and planning of terrorist attacks in Baghdad and other places,” said Hussein al-Asadi, a lawmaker with Maliki’s bloc. “The judicial authority has issued arrest warrants against those who are involved.”

Iraqiya leaders linked their walkout directly to the timing of the American withdrawal, which, they said, had left Maliki’s rivals vulnerable to the predations of an army and police force that the Shiite prime minister has increasingly brought under his personal control over the past year.

The U.S. military formally declared the Iraq war over at a ceremony outside Baghdad on Thursday, and the last few hundred soldiers crossed the border into Kuwait early Sunday morning.

“We think there are new indications of a new attempt to create a dictatorship,” said Deputy Prime Minister Saleh al-Mutlaq. “We are really worried that the country is being led into chaos and division and the possibility of civil war is there.”

A brewing confrontation in the province of Diyala underscored the risk that violence could erupt. After the mostly Sunni leadership of the province declared last week that it intends to seek regional autonomy under the terms of Iraq’s constitution, Shiite militiamen surrounded the provincial council headquarters and set fire to the Sunni governor’s home.

The governor and most members of the provincial council have fled to northern Kurdistan, and on Saturday, the main highway linking Baghdad to the northern city of Kirkuk was blocked for a third day by Shiite militiamen who, residents said, belong to Moqtada al-Sadr’s Mahdi Army.

The crisis marks the most serious breakdown yet of the consensus forged a year ago between the main Shiite, Sunni and Kurdish political blocs that enabled the creation of the current coalition government. For the first time, the Sunni Iraqiya bloc, which won the largest number of votes in last year’s election, was given meaningful positions in the government.

But tensions have been building for months between the factions over Maliki’s failure to include Sunnis in the decision-making process and his steady consolidation of personal control over the security forces. He has retained the positions of defense and interior ministers for himself, and used the de-Baathification laws drawn up by the American occupation authority in 2003 to replace thousands of Sunni officers as well as independent Shiites with his own loyalists.

The detentions in October of hundreds of suspected sympathizers of Saddam Hussein’s former Baath Party, many of them Sunni, have fueled a push for regional autonomy by the mostly Sunni provinces to the north and west of Baghdad, which Maliki has vowed to resist.

Sunnis in the provinces say they fear persecution both by the Shiite government and Sunni extremists now that U.S. troops are no longer present.

Gen. Khaled al-Dulaimi, who helped U.S. forces establish the Anbar Police Academy in 2007, was stripped of his post last month as U.S. troops were pulling out of the western province. He predicted that many other officers will be sidelined now that U.S. troops have gone. The U.S. military built the Anbar security forces almost from scratch after the Sunni Awakening movement in 2007 succeeded in defeating the al-Qaeda in Iraq insurgency.

And those who collaborated with the Americans are also at risk of being targeted by the remnants of the Sunni al-Qaeda fighters, who have been systematically pursuing those who turned against them. Now that he has been stripped of the security that came with his position, Dulaimi said, “I might be assassinated by terrorists at any time.” He added, “Who is going to protect me?”

Special correspondents Asaad Majeed and Aziz Alwan contributed to this report.