JAKARTA, INDONESIA — As a boy in Indonesia, Senator Obama crisscrossed the religious divide. At the local primary school, he prayed in thanks to a Catholic saint. In the neighborhood mosque, he bowed to Allah.

Having a personal background in Christianity and Islam might seem useful for an aspiring American president in an age when Islamic nations and radical groups are key national security and foreign policy issues. But a connection with Islam is untrod territory for presidential politics.

Mr. Obama’s four years in Indonesia underscore how dramatically his background differs from that of past presidential hopefuls, most of whom spent little, if any, time in other countries. No one knows how voters will react to a candidate with an early exposure to Islam, a religion about which many Americans know very little.

Mr. Obama’s campaign aides have emphasized his strong Christian beliefs and downplayed any Islamic connection. The candidate was raised “in a secular household in Indonesia by his stepfather and mother,” his chief spokesman, Robert Gibbs, said in a statement in January after reports began circulating that Mr. Obama had attended a madrassa, or Koranic school, as a child.

“To be clear, Senator Obama has never been a Muslim, was not raised a Muslim, and is a committed Christian who attends the United Church of Christ in Chicago,” Mr. Gibbs’s January 24 statement said.

His Roman Catholic and Muslim teachers, along with two people who were identified by Mr. Obama’s grade-school teacher as childhood friends, say Mr. Obama was registered by his family as a Muslim at both schools he attended. That registration meant that during the third and fourth grades, Mr. Obama learned about Islam for two hours each week in religion class.

The childhood friends say Mr. Obama sometimes went to Friday prayers at the local mosque. “We prayed but not really seriously, just following actions done by older people in the mosque,” Zulfin Adi said. “But as kids, we loved to meet our friends and went to the mosque together and played.”

The campaign’s national press secretary, Bill Burton, said Wednesday that the friends were recalling events “that are 40 years old and subject to four decades of other information.” Mr. Obama’s younger sister, Maya Soetoro, said in a statement released by the campaign that the family attended the mosque only “for big communal events,” not every Friday.

The report that Mr. Obama had attended a radical madrassa here, appearing initially on a conservative-oriented online magazine and then on a Fox News program, attributed the news to opposition researchers for Senator Clinton. Both campaigns denied the story and accused conservative news outlets of trying to use the rumor to smear two Democratic hopefuls simultaneously.

In his autobiography, “Dreams from My Father,” Mr. Obama briefly mentions Koranic study and describes his public school, which accepted students of all religions, as “a Muslim school.”

“In the Muslim school, the teacher wrote to tell my mother that I made faces during Koranic studies,” Mr. Obama wrote. “My mother wasn’t overly concerned. ‘Be respectful,’ she’d say. In the Catholic school, when it came time to pray, I would close my eyes, then peek around the room. Nothing happened. No angels descended. Just a parched old nun and 30 brown children, muttering words.”

Mr. Obama was born in Honolulu. When he was 2, his father, Barack Hussein Obama Sr., a Kenyan, and his Kansas-born mother, Ann Dunham, separated and later divorced. Ms. Dunham later married Lolo Soetoro, who was a Muslim. In 1967, the family moved to Jakarta, where Mr. Obama lived between ages 6 and 10. People there knew him as Barry Soetoro.

In 1968, Mr. Obama began first grade at St. Francis Assisi Foundation School, just around the corner from his home.