Yaser ElMenshawy never saw the man's face, but he doubts he will ever forget him. At a fund-raiser last year for a new Brooklyn mosque, the master of ceremonies asked if anyone would donate $100,000 to kick off the campaign. A man's voice called out from the crowd: Yes!

"And to top it off, he wrote a check" on the spot, said ElMenshawy, a software engineer from Woodbridge, N.J., and chairman of a network of Islamic organizations in his state. "That guy impressed me."

Not every American Muslim has an extra hundred grand sitting in a checking account, to be sure, but in these heady days of economic prosperity, many appear to be opening their wallets like never before. They are giving to erect mosques, Islamic community centers, and schools across America; to help needy Muslims at home and overseas; to support non-sectarian institutions such as universities and hospitals; and to help America's urban poor, regardless of creed.

Tally it all up, and some experts estimate the total would top $10 billion annually, said Kareem Irfan, a spokesman for the Indiana-based Islamic Society of North America, a national umbrella group. Dr. Sayyid M. Syeed, ISNA secretary-general, says Muslim participation in the "cyber- revolution," as well as a strong economy, has fueled much of this increase.

At the root of this philanthropy, however, is faith. Zakat, "charity," the third of Islam's "five pillars," is outlined in the very beginning of the Qur'an. Traditionally, that obligates each Muslim to donate 2.5% of his or her annual savings, making needy family members the first priority. Add to that the principle of sadaqa, optional charity, and you have a religion where generosity is as fundamental as daily prayer and faith in God.

Exact figures are not easy to track. Faith-based nonprofits saw donations increase 5.5% last year, to $81.78 billion, according to estimates by the American Association of Fundraising Counsel, which does not specifically track Muslim charitable giving. Unlike the major Christian or Jewish charities, which rely on local offices and foundations to channel funds to various causes, America's Islamic charities have yet to build such sophisticated networks.

Many believe it's simply a matter of time before America's Muslim community, now estimated at between 6 million and 10 million strong, will build institutions rivaling those built by Christians and Jews.

"Our community is a young community. Our religious scholars, there are none here. We're having to bring them in from overseas," said Dalell Mohammed, spokeswoman for the Texas-based Holy Land Foundation, one of the nation's most established and popular Islam-based charities. "I think once the American Muslims start coming up in this field, I think it will be much, much more organized and more connected and more unified."

In some cases, Muslim charities are hindered more by politics than lack of professional polish.

The Holy Land Foundation took in more than $6 million last year, most of it directed to humanitarian efforts in Palestine, Lebanon, and Jordan, as well as to needy Muslims in Turkey, Kosovo, and Bosnia. Recent American and Israeli investigations into the charity's activities in Palestine have frightened some donors, Mohammed said, but she is optimistic that American Muslims will continue to support what she says is the foundation's primary focus: helping Muslims in need, wherever they may live.

The seven-year-old Global Relief Foundation of Chicago, another popular Islam-based charity, helps the poor in 17 nations, including Kosovo, Lebanon, Pakistan, and Georgia. Although nonprofit and nonpolitical, they have been unable to gain access to certain regions, said Farooq Burney, a GRF spokesman.

"Iraq is a place where we have not been able to work," Burney said. "Obviously, when you see there's so much suffering going on, it does bring a certain amount of frustration."

Another challenge for Muslim charities is the fact that many donors want to see quick results, an issue faced by many nonprofit groups seeking funds.

"People tend to give money where they see the money spent, more so than to some international organization that is going to be `over there' somewhere," ElMenshawy said. One result has been the phenomenal growth in community infrastructure.

"All across the nation, especially in Michigan, Illinois, California, and Houston, we're seeing lots and lots of mosques," he said, adding that more than 60 mosques have been organized in New Jersey in the past 25 years. "On the negative side, that has been a real drain for the community, because when you put money into one thing that means you put less money into something else. There are not a lot of social-service institutions where large donors would show up."

The emphasis on houses of worship, rather than hospitals or universities, is no fluke, however.