Crowds began arriving in Mashhad late Wednesday, streaming into the holy city from across Iran and abroad. Long after midnight, the streets around the shrine were packed with people waving Iranian flags, chanting religious slogans and commemorating the life of Ayatollah Khamenei.

Inside the Imam Reza Shrine, one of the holiest sites in Shiite Islam, thousands of worshipers kept vigil through the night. Some recited verses from the Quran while others prayed quietly in groups. Families spread blankets and thin carpets across the shrine’s vast courtyards and nearby sidewalks, sleeping where they could as they waited for the funeral ceremonies to begin at daybreak.

As the sun rose, the crowds swelled. Imam Reza Boulevard, the main avenue leading to the shrine, was closed to traffic and transformed into a sea of mourners. With temperatures climbing quickly, many sought shelter beneath tents or in narrow strips of shade cast by buildings and trees. Volunteers distributed tea and bottled water as giant screens broadcast live images from inside the shrine.

Iran’s leaders hoped the funeral would serve as a show of unity to rally supporters and warn adversaries and potential dissidents of its strengths, said Ali Ansari, a professor of Iranian history at the University of St. Andrews.

But he said that the resumption of hostilities undermined the Iranian government’s narrative that it had won the war, a message it wanted to reinforce with the funeral. “They wanted to present it as resolved in their favor,” he said.

On Wednesday, Mr. Trump had told reporters at the NATO summit in Ankara, Turkey, that the Iranians authorities had requested a “time out” from the conflict during the funeral proceedings.

Earlier this week, though, the long-simmering tensions over shipping in the Strait of Hormuz boiled over. Both Iran and the United States said the strait would reopen following last month’s preliminary cease-fire deal, but Iran has sought to assert its control over the critical waterway and U.S. officials demanded that it return to its prewar status. This week Central Command accused Iranian forces of attacking commercial ships in the strait and said it hit almost 100 Iranian military targets in response.

The gatherings in Mashhad underscored both the religious and political significance of Ayatollah Khamenei’s death. Pilgrims and mourners had traveled not only from across Iran but also from countries with large Shiite populations, including Iraq, Pakistan and India, as well as Senegal and Nigeria. For many, the funeral marked the passing of one of the most influential figures in modern Shiite Islam.

Shawn McCreesh contributed reporting from Ankara, Turkey, and Sanam Mahoozi contributed reporting from London.