Updated

Security forces are fanning out across Lebanon after Sunni lawmakers called for a nationwide "day of rage" to protest Hezbollah's gains in the government.

The Iranian ally has moved to the brink of controlling the country's next government, setting off protests and drawing warnings from the U.S. that its support could be in jeopardy.

The Shiite militant group — considered a terrorist organization by Washington — secured support in parliament Monday to name its own candidate for the next prime minister.

Hezbollah's Sunni rivals planned protests Tuesday in the northern city of Tripoli, Beirut and other parts of Lebanon.

The U.S. says continuing American support for Lebanon would be "problematic" if Hezbollah takes a dominant role in the government.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

BEIRUT (AP) — Iranian ally Hezbollah moved to the brink of controlling Lebanon's next government on Monday, setting off angry protests and drawing warnings from the U.S. that its support could be in jeopardy.

Nearly two weeks after bringing down Lebanon's Western-backed government, the Shiite militant group — considered a terrorist organization by Washington — secured support in parliament to name its own candidate for the next prime minister. The feat caps Hezbollah's steady rise over decades from resistance force against Israel to Lebanon's most powerful military and political power.

Protests erupted quickly in areas populated by Hezbollah's Sunni rivals, who declared a "day of rage" Tuesday to express their rejection of what they called "Persian tutelage" over Lebanon — a reference to Hezbollah's Iranian patrons.

Hezbollah's candidate, billionaire businessman Najib Mikati, was set to clinch the nomination after Hezbollah and its allies lined up the needed backing of at least 65 of the 128 parliament members as voting began Monday.

Hezbollah's Western-backed opponents maintain having an Iranian proxy in control of Lebanon's government would be disastrous and lead to international isolation. The United States, which has poured in $720 million in military aid since 2006, has tried to move Lebanon firmly into a Western sphere and end the influence of Hezbollah, Syria and Iran.

State Department spokesman P.J. Crowley warned Monday that continuing U.S. support for Lebanon would be "problematic" if Hezbollah takes a dominant role in government, though he declined to say what the U.S. would do if Hezbollah's candidate becomes prime minister.

A Hezbollah-led government would also raise tensions with Israel, which fought a devastating 34-day war against the Shiite militants in 2006 that left 1,200 Lebanese and 160 Israelis dead. Hezbollah briefly took control of Beirut's streets two years later in sectarian clashes that killed 81 people, angering many who accused the militants of breaking a promise to never use its arsenal against the Lebanese.

Then in 2009, the Shiite militant group joined the government with virtual veto power over all its decisions.

Hezbollah brought down that government on Jan. 12 after Prime Minister Saad Hariri refused the group's demand to cease cooperation with a U.N.-backed tribunal investigating the 2005 assassination of his father, former Prime Minister Rafik Hariri.

Hezbollah can either form its own government now, leaving Hariri and his allies to become the opposition, or it can try to persuade Hariri to join a national unity government. In a speech Sunday night, Hezbollah leader Sheik Hassan Nasrallah said he favored a unity government.

Hariri said Monday he will not join a government headed by a Hezbollah-backed candidate.

Hariri's coalition issued a statement last week saying Hezbollah is trying to turn Lebanon into an "Iranian base" and was using intimidation to get its way. Hezbollah has emphasized that the group brought down Lebanon's government democratically and without resorting to violence.

Hezbollah keeps a massive arsenal that outweighs that of Lebanon's national army, saying it needs the weapons to ward off any threats from Israel. But the movement's reputation has taken a hit in recent years among those who see it as dragging the country into violent conflicts.

Several hundred Hariri supporters in the northern city of Tripoli, a predominantly Sunni area and a hotbed of fundamentalists, staged protests Monday chanting anti-Mikati slogans.

The protesters carried pictures of Hariri, shouting, "Mikati you are not one of us, leave and go away." Some carried banners that read: "The blood of Sunnis is boiling."

In the eastern Bekaa Valley, witnesses said the army fired tear gas to dispel protesters.

Despite the strident opposition from the Hariri camp, Mikati is seen as a relatively neutral figure who enjoys good relations with both Syrian President Bashar Assad and with Hariri — putting the latter in the awkward position of having to openly reject a candidate who has been an ally in the past.

Mikati emphasized Monday that he would represent all of Lebanon, even as he insisted he would safeguard "the achievements of the national resistance," a reference to Hezbollah.

"I don't distinguish between anyone," said Mikati, a Harvard graduate and businessman whose wealth is estimated at $2.5 billion. "I extend my hand to everyone without exception ... I say to Prime Minister Saad Hariri, let us all work together for the sake of Lebanon."

It is significant that Hezbollah chose a relatively centrist candidate for prime minister — as opposed to a staunchly pro-Syrian one, such as Omar Karami — even though the group has secured enough power to govern on its own.

The move indicates that Hezbollah is at least paying lip service to the idea that a unity government could be formed. It also corners Hariri, who will have to reject an ally.

Since Hezbollah and its allies forced the government's collapse by resigning from the Cabinet, both sides have been scrambling for the support of at least 65 lawmakers, the required number to form a government in Lebanon's 128-seat Parliament. Hezbollah crossed the finish line first Monday, and voting was to continue on Tuesday.

Lengthy negotiations and an extended political deadlock could lie ahead as Mikati seeks to win over Hariri's bloc into a unity government.

Oqab Saqr, a lawmaker allied with Hariri, all but conceded defeat Monday.

"We may have lost the prime ministry but we will win the country and we will win justice," he told reporters after he voted for Hariri for the post Monday.

Mikati, who served briefly as premier in 2005, overseeing the first parliament elections following the withdrawal of Syrian troops from Lebanon, says he is seeking the post as a candidate of "moderation and accord."

But he dodged a question about whether he would end Lebanon's cooperation with the international court — a key Hezbollah demand — saying only that "any dispute can be solved only through dialogue."