BEIRUT, Lebanon — Hezbollah’s leader, Hassan Nasrallah, called on Sunday for “patience and calm” in Lebanon, seeking to reassure a country thrown into uncertainty a day after the surprise resignation of the prime minister, Saad Hariri.

Fears for Lebanon’s stability were running high after Mr. Hariri declared his resignation in a speech televised from Riyadh, the Saudi capital, attacking Saudi Arabia’s regional rival Iran and its ally Hezbollah. The Shiite militia and political party is the strongest faction in Lebanon.

The move was widely seen as having been orchestrated by Mr. Hariri’s patrons, the Saudis, to isolate Hezbollah by collapsing Lebanon’s national unity government, which included both it and Mr. Hariri’s Sunni faction. Saudi Arabia has been taking increasingly aggressive steps to curb Iran’s growing dominance in the region.

Lebanon has long been a flash point for regional tensions, and there are fears that if it becomes the latest stage for the escalating rivalry between Saudi Arabia and Iran — as war-torn Syria has been in recent years — the tiny Mediterranean country could face economic collapse or even violence.