WASHINGTON — With the holy month of Ramadan set to begin today in much of the Islamic world, religious leaders from Morocco to Indonesia are urging the world's more than 1 billion Muslims to focus on prayer, repentance and charity.

But fears of attacks by extremists keyed to the holiday are raising concerns, especially in Iraq, where the start of Ramadan was one of the bloodiest days since the insurgency began after the U.S.-led invasion and occupation in 2003.

Last year, Ramadan began with a series of car bombings in Baghdad. One day earlier, Iraqi insurgents killed 16 Americans when they shot down a U.S. military transport helicopter. Other attacks during the holy month in 2003 included strikes in Turkey and Saudi Arabia.

In the United States, the holiday comes as authorities are already on alert, fearing an al-Qaida attack before the Nov. 2 presidential election.

For Muslims, Ramadan is considered the most arduous and uplifting time of the year. It commemorates God's revelation of the Quran, the Muslim holy book, to the Prophet Muhammad some 1,400 years ago.

During the month, Muslims abstain from food, drink and sex from dawn to sunset. The abstinence is intended to downplay one's physical needs and focus instead on spirituality. Deprivation also is meant to heighten compassion for the less fortunate, who feel hungry and thirsty throughout the year.

In the weeks leading up to Ramadan, Muslim leaders across the Middle East and elsewhere have urged their followers to use the time to reflect on their lives and how to become better people.

"This is a chance for the umma (the worldwide Muslim community) to purify itself," said Sheik Ismail Nawahida, a leading imam, or prayer leader, at Jerusalem's Al Aqsa Mosque, considered the third holiest shrine for Muslims. "It is a time to restore balance . . . (and) to remind ourselves about the basics."

In recent years, extremists have chosen the holiday to attack Western targets, some apparently driven by the belief that to fight a non-Muslim enemy during the holy month earns a special place in heaven. In Iraq, U.S. forces have been striking at insurgents in an effort to limit attacks during Ramadan.

The first major battle in Islam's history came during Ramadan. The Battle of Badr in 624 A.D. was Muhammad's first military victory and is credited with making Islam a major force in the region. Muhammad's much smaller force prevailed in what Muslims view as a divine event.

But some experts cautioned about drawing a link between Ramadan and attacks against non-Muslims.

Alan Godlas, a professor of religion at the University of Georgia who teaches about Islam, said terrorists who struck Western targets last year during Ramadan must have had other considerations.