SALT LAKE CITY — Thousands of believers were in mourning today following the death of Gordon B. Hinckley, the humble head of the Mormon church who added millions of new members and labored long to burnish the faith’s image as a world religion. An announcement of his successor was not expected for days.

Hinckley, the 15th president of The Church of Jesus Christ of Latter-day Saints, died yesterday of complications arising from old age, a church spokesman, Mike Otterson, said. He was 97.

“His leadership in humanitarian efforts around the world was matched only by his efforts in his own beloved state and community as a committed citizen,” Governor Huntsman of Utah, a Mormon, said. “He has stood as a remarkable example of selflessness, charity and humility and he will be greatly missed by all.”

By tradition, at a church president’s death, the church’s most senior apostle is ordained within days on a unanimous vote of the Council of the Twelve Apostles. The longest-serving apostle now is Thomas S. Monson, 80.

The vote is not likely to occur until after Hinckley is laid to rest. At least twice in the past the naming of a new president has lagged for several years, but in modern times the announcement has come within a week.

Hinckley, a grandson of Mormon pioneers, was president for nearly 13 years. He took over as president and prophet on March 12, 1995, and oversaw one of the greatest periods of expansion in church history. The number of temples worldwide more than doubled, from 49 to more than 120 and church membership grew from about 9 million to about 13 million.

Mitt Romney, who is trying to become the first Mormon elected president, said today he would miss the humility and wisdom of Hinckley and plans to attend his funeral.

The former Massachusetts governor recalled meeting Hinckley both when he served as president of the Salt Lake Organizing Committee for the 2002 Winter Olympic Games, as well as before he launched his White House bid. Mr. Romney said Hinckley was pleased he was endeavoring to become the first Mormon elected president.

“He smiled and said it would be a great experience if you won and a great experience if you lost,” Mr. Romney said.

The church presidency is a lifetime position. Before Hinckley, the oldest church president was David O. McKay who was 96 when he died in 1970.

Hinckley became by far his church’s most traveled leader in history. And the number of Mormons outside America surpassed that of American Mormons for the first time since the church, the most successful faith born in America, was founded in 1830.

Dozens of mourners gathered last night outside Mormon church headquarters to honor Hinckley. College students sang hymns by the light of their cell phones.

Genoba Urbina recalled how humble Hinckley appeared when she served food and drinks at a conference in New Orleans four years ago. He even asked for her lemonade recipe.

“He didn’t want anything special. He danced and wore Mardi Gras beads,” Ms. Urbina, of North Salt Lake, said. “He was so loving.”

Hinckley had been diagnosed with diabetes and was hospitalized in January 2006 for the removal of a cancerous growth in his large intestine. He later resumed a regular work schedule and his last public appearance was January 4

About 62% of Utah’s 2.7 million residents are members of The Church of Jesus Christ of Latter-day Saints. Leaders in all levels of government are members, including the governor, lieutenant governor, and attorney general.

Hinckley worked to show that his faith was far removed from its peculiar and polygamous roots. Still, during his tenure, the Roman Catholic Church, Southern Baptist Convention, and United Methodist Church — the three largest American denominations — each declared that Mormon doctrines depart from mainstream Christianity.

“The more people come to know us, the better they will understand us,” Hinckley said in an interview in late 2005. “We’re a little different. We don’t smoke. We don’t drink. We do things in a little different way. That’s not dishonorable. I believe that’s to our credit.”