BEIRUT - The Shiite armed movement Hezbollah cobbled together enough support Monday to appoint Lebanon's next prime minister, effectively ending nearly six years of rule by Western-backed leaders and prompting the United States to warn it could cut off aid to this key Arab nation.

The apparent strength of Hezbollah marked an important political achievement for the group. It also signaled what appeared to be a significant shift for the country, away from alliances with the United States and Saudi Arabia, and toward Iran and Syria, which support Hezbollah.

In Washington, State Department spokesman P.J. Crowley said the United States "would have great concerns about a government within which Hezbollah plays a leading role."

The United States considers Hezbollah a terrorist organization. A prominent government role for the group could compel the U.S. government to list Lebanon as a terrorist state. That designation would carry with it the automatic suspension of aid, a possibility that Crowley acknowledged. The United States has sent about $1.2 billion in economic and military assistance to Lebanon in the past five years, and the Obama administration has requested $246 million more this year.

Hezbollah and its allies agreed Monday to support Najib Mikati, a Sunni self-made billionaire, for the prime minister's post, a decision that could be confirmed by the full parliament as early as Tuesday.

But even as Hezbollah worked to install its candidate as prime minister, the group tried to play down its role in the new government. Hassan Nasrallah, the group's leader, said in a speech Sunday that Hezbollah hoped to form a national unity government with broad participation. "We are not seeking authority," he said.

Hezbollah has said repeatedly that it does not want to rule Lebanon. Instead, the group forced a change in government to end Lebanon's cooperation with a United Nations tribunal investigating the 2005 assassination of Rafik al-Hariri, a former prime minister. Sealed indictments were issued by the tribunal last week, and are widely thought to name several members of Hezbollah.

But the reaction from the group's Sunni opponents left no doubt that, for them, such a government would represent a dangerously strengthened hand for Hezbollah, already the most powerful military force in the country, and for its backers in Iran.

Saad al-Hariri, who served as prime minister since November 2009, said Monday he would not participate in a government led by a Hezbollah-backed candidate. Hariri's backers called for a "day of anger" on Tuesday, and complained about what they said was Iranian meddling in Lebanon.

They gave a taste of that anger Monday, protesting in the streets of the mostly Sunni northern city of Tripoli, cursing Mikati and the head of Hezbollah, burning tires and closing down a vegetable market, local television reported. In swaths of the north and Beirut, protesters stopped cars, burned tires and blocked roads as the Lebanese army rushed to control them.

The possibility of a Hezbollah-dominated government also prompted concern in Israel, which fought a war with Hezbollah in 2006.

Silvan Shalom, an Israeli vice prime minister, called the possibility of a government dominated by Hezbollah "a very, very dangerous development," telling a weekend radio interview it would be like having "an Iranian government on Israel's northern border."