BAGHDAD — Iraq's political crisis deepened Sunday as Prime Minister Nouri al Maliki ordered the country's vice president off of a plane and had him held temporarily at Baghdad airport, on suspicion that members of his security detail took part in a string of assassinations.

The confrontation between the prime minister, a Shiite Muslim, and Vice President Tariq al Hashimi, a Sunni, took the spotlight off of what could have been a day of Iraqi unity and celebration — as the last American tanks and troops rolled south to Kuwait, ending the nearly nine-year U.S. military presence.

In Baghdad's Green Zone, the center of the U.S. occupation following the 2003 invasion, the tanks, personnel carriers and hundreds of troops that took up positions this weekend were Iraqi, not American. But their intimidating presence symbolized the sudden political crisis as well as the shift in power.

Maliki's move against Hashimi followed the arrest of at least six members of the vice president's security detail in the past two weeks, Interior Ministry officials said. On Friday, Maliki told members of his ruling multi-party alliance that several of the guards had confessed involvement in the assassinations of Shiite politicians over the past two years, according to a participant at the session. Hashimi's security detail is said to number 300.

Hashimi and several other Sunni politicians were about to fly to Kurdistan Sunday evening for dinner with Kurdish leader Massoud Barzani when Maliki's head of military intelligence ordered them not to depart. According to one account, from a leading politician who declined to be identified due the sensitivity of the issue, Hashimi, vice premier Saleh al Mutlaq and Finance Minister Rafie al Essawi were already aboard the plane when they were ordered to disembark.

According to another account, from a source in the Interior Ministry who spoke on condition of anonymity because he wasn't authorized to be quoted, the entire airport was shut down while the men were detained. Hashimi was told that the authorities had no direct case against him, but were prepared to charge that he had been an accomplice in smuggling some of the assassins out of Baghdad. Security authorities arrested two of Hashimi's bodyguards, the Interior Ministry official said.

After a flurry of phone calls involving political figures from nearly every party, Maliki relented and allowed the men to continue the journey.

It was unclear whether Hashimi, one of Iraq's two vice presidents, could in fact be arrested, unless the Iraqi parliament first strips him of immunity. But if the accounts are correct that some of his security guards confessed to taking part in the assassinations of prominent Shiites, his days in national politics may be numbered.

In a second crisis, tangentially related to the case against Hashimi, Maliki formally asked the Iraqi parliament on Sunday to vote no confidence in Mutlaq, after the vice premier called the prime minister "a dictator" during a CNN television interview.

The Iraqiya bloc, which shares power with the Maliki's State of Law alliance and others, announced Saturday that it would boycott parliamentary sessions on the grounds that it has been "marginalized" by Maliki. The move by Iraqiya leader Ayad Allawi, Maliki's chief political rival, came a day after Maliki told political leaders that he would seek Mutlaq's ouster.