BEIRUT, Lebanon — A week after the Lebanese prime minister, Saad Hariri, flew to Saudi Arabia and announced his resignation, what seemed at first like a bizarre domestic political dispute is escalating tensions in the Middle East and threatening to become a flash point in the struggle for power there.

On Friday, Mr. Hariri remained stranded in Saudi Arabia. The Iranian-backed militia Hezbollah said the Saudis were holding him against his will, while the Saudis have said there was a plot to assassinate him.

Fractious Lebanese politics and interference in them by Saudi Arabia, Iran and a host of other powers are nothing new, but the Hariri case has become part of a high-stakes buildup of tension that is fueling anxiety about whether the region is on the verge of war.

The United States on Friday urged calm, with Secretary of State Rex W. Tillerson warning “against any party, within or outside Lebanon, using Lebanon as a venue for proxy conflicts or in any manner contributing to instability in that country,” a message apparently aimed at Hezbollah as well as Saudi Arabia.

As events in the region have unfolded over the past week, each more surprising and bewildering than the last, world leaders, analysts and diplomats have scrambled to figure out what is behind them and whether they are all connected, and even, as some analysts fear, whether they are part of a broader plot to spark a war between Israel and Hezbollah that would risk a wider conflagration.

First, a week ago, Mr. Hariri unexpectedly flew to Riyadh, the Saudi capital, without any of his close advisers. A day later, he announced his resignation on Saudi television, something he had shown no signs of planning to do.

Hours later, on Saturday evening, a missile fired from Yemen came close to Riyadh before being shot down. Saudi Arabia later blamed Iran and Hezbollah for the missile, suggesting that they had aided the Iran-aligned Houthi rebels in Yemen to fire it.

Before the world had a chance to absorb this news, the ambitious and aggressive Saudi Arabian crown prince, Mohammed bin Salman, ordered the arrest of hundreds of Saudis, including members of the royal family, in what is either a crackdown on corruption, as Saudi officials put it, or a political purge, as outside analysts have suggested.

It then emerged that the week before, Jared Kushner, President Trump’s son-in-law and adviser, who has been sent on missions both to Israel and Saudi Arabia, had visited Riyadh on a previously undisclosed trip and met until the early morning hours with the crown prince.

Even before these events unfolded, analysts and officials in Lebanon and around the region had been increasingly anxious about a volatile combination: an impulsive, youthful Saudi leader escalating threats to roll back growing Iranian influence, an equally impulsive Trump administration signaling broad agreement with Saudi policies, and increasingly pointed warnings from Israel that it may eventually fight another war with Hezbollah.

Some Israeli officials have said over the past year, both publicly and in meetings in Washington, that Hezbollah had grown too powerful, that it was time to strike a decisive blow against it, and that they were preparing for war — even as others insisted the country did not want a conflict now.

On Friday afternoon, Hezbollah’s leader, Hassan Nasrallah, declared in a televised speech that Saudi Arabia had asked Israel to attack Lebanon, after essentially kidnapping Mr. Hariri, who had been part of a unity government that included Hezbollah.

“I’m not talking here about analysis, but information,” he said. “The Saudis asked Israel to attack Lebanon.”

He provided no evidence of his claim, but Western and regional analysts have also said that, given all the confusing and unexpected events and unpredictable players, they could not entirely rule out such a scenario.

There have long been fears that now that the Syrian war — in which Hezbollah played a decisive role, gaining new influence, power and weapons — is almost over, Hezbollah’s enemies might seek to cut it down to size, and that an empowered Hezbollah would push back hard.

Addressing Saudi Arabia, Mr. Nasrallah said, “Don’t set a goal of destroying Hezbollah because you can’t.” But if Saudi Arabia’s goal was to force Hezbollah to leave Syria, he said: “No problem. Our goal there has been achieved. It’s almost over anyway.”

Also on Friday, President Emmanuel Macron of France left Saudi Arabia after a brief, last-minute meeting with the crown prince.

During the unexpected two-hour visit on Thursday, tacked on to a trip to Abu Dhabi to open a new branch of the Louvre museum there, Mr. Macron “reiterated the importance France attaches to Lebanon’s stability, security, sovereignty and integrity,” the French president’s office said. He also discussed “the situation in Lebanon following the resignation of Prime Minister Hariri,” his office said, but provided no further details.

At a news conference in Dubai, the United Arab Emirates, before the meeting, Mr. Macron said he did not share Saudi Arabia’s “very harsh opinions” of Iran.

“It is important to speak with everyone,” Mr. Macron said.

France has longstanding interests in Lebanon, a former colony, and Mr. Macron was invited to Saudi Arabia by Prince Mohammed.

A new war is unlikely, analysts said. Saudi Arabia, which is mired in a military conflict in Yemen, is seen as lacking the capacity to start another, while Israel, which has talked about reducing the new influence of Hezbollah in Lebanon and the region, has said it does not want a war now.

But some have warned that the increased tensions could provoke an economic crisis or even war accidentally. Miscalculations have started wars before, as in the 2006 war between Israel and the Lebanese Shiite militant group Hezbollah.

In Lebanon, concerns have mounted that Mr. Hariri may, in effect, be a hostage of the Saudi government.

President Michel Aoun of Lebanon told the Saudi chargé d’affaires in Beirut, Walid Bukhari, that the manner of Mr. Hariri’s resignation was “unacceptable,” and a consortium of countries and organizations interested in Lebanon’s stability said they had met on Friday with Mr. Aoun, who called for Mr. Hariri’s return.

Mr. Hariri said in his resignation speech that he was quitting because of what he said was Iran’s disproportionate influence in Lebanon through its ally, Hezbollah, which is part of the unity government he headed. But he had shown no signs of planning to resign before his sudden trip to Riyadh, and so far the political fallout has harmed him and his party more than Hezbollah.

The consortium, the International Support Group for Lebanon, issued a statement expressing “concern regarding the situation and prevailing uncertainty in Lebanon” and calling for Lebanon to be “shielded from tensions in the region.”

The group’s members — including the United Nations, Britain, China, France, Germany, Italy, Russia and the United States, as well as the European Union and the Arab League — are not all on the same side of some issues in the region, so the statement seemed to reflect a high level of concern about tensions.

Mr. Hariri, a dual citizen of Lebanon and Saudi Arabia, met the ambassadors of Britain and the European Union and the chargé d’affaires of the American Embassy on Wednesday and Thursday at his Riyadh residence.

Other Western diplomats, speaking on the condition of anonymity, said those envoys, too, had the impression that Mr. Hariri could not speak freely. Saudi Arabia, which ordered its citizens on Thursday to leave Lebanon, has denied he is being held against his will.

Correction: November 10, 2017

Because of an editing error, an earlier version of this article misstated where President Emmanuel Macron of France met Crown Prince Mohammed bin Salman of Saudi Arabia. They met in Riyadh, not in Abu Dhabi.