The sexual exploitation of girls and boys, largely by American men, has reached alarming proportions in Central America, according to children's rights advocates who say the region is now a priority in their struggle against child prostitution and pornography.

A major reason for growth in the Central American child-sex trade, children's advocates say, is that traditional destinations for such activity--chiefly Thailand and the Philippines--have blunted the sex tourism business over the last two years by enacting public awareness campaigns and stricter laws and enforcement measures.

Prostitution among the children who live and work on the streets of Latin America--their number has been estimated at up to 40 million--has long been a consequence of the region's poverty. But as such countries as Guatemala, El Salvador, Costa Rica and Nicaragua step up efforts to promote their beaches, volcanoes and natural beauty as tourist destinations, they attract greater numbers of men from North America, Europe and other Latin American countries looking for sex with children.

"What we are seeing is the dark side of tourism," said Heimo Laakkonen, the head of UNICEF in Costa Rica. Laakkonen said that while sexual exploitation of minors is not a new problem in the region, "with the increase in tourism, the problem has gotten worse."

Sitting at the bar in the dingy Del Ray Hotel here one recent evening, a 33-year-old California bartender named David said he was on his second trip to Costa Rica in as many years. He spoke brazenly about how he had scanned several Web pages advertising youthful-looking female prostitutes in Costa Rica in his efforts to have sex with a girl who had no previous sexual experience.

David, a stocky, unkempt man who insisted that only his first name be used, boasted of how he had arranged for one of the many taxi drivers connected with the sex trade to bring a 13-year-old girl from her parents' home in a poor San Jose neighborhood to his hotel. The girl's mother and father asked for $400 for use of the girl, which David said he eagerly paid.

Costa Rican law allows only women 18 and older to work as prostitutes. Stiffer penalties enacted recently threaten prison terms of up to 10 years for anyone convicted of buying sex from a minor. The prospect did not seem to alarm David.

"I am living out a fantasy . . . and nobody looks like they have a real problem with it," he said. As he spoke, adult prostitutes mingled with foreigners in the hotel lobby as younger ones strolled the streets outside.

Costa Rica, which in 1999 drew more than 1 million foreign visitors for the first time, is Central America's leading tourist destination. It is also believed to have the region's most pronounced child-prostitution problem.

Children's rights activists have accused governments in Central America, where about 54 percent of the population is below the age of 18, of being slow to confront the region's burgeoning child prostitution and pornography industry. "It involves a certain level of political maturity on the part of governments to acknowledge the severity of the problem, as opposed to the ostrich syndrome of keeping your head stuck in the sand," said Bruce Harris, regional director of Covenant House (Casa Alianza) Latin America, an organization that helps street children.

Although there are no statistics to quantify the scope of sexual exploitation of children in Central America, anecdotal evidence, independent surveys and a string of recent arrests of Americans--as well as of other foreigners and locals--support the contention that the problem is growing.

The increased demand for child prostitutes in this region and others stems partly from the mistaken impression that older prostitutes are more likely than younger ones to have AIDS or carry the HIV virus, experts say.

Carlos Roverssi, the former executive president of Costa Rica's National Child Trust, the government's child welfare agency, acknowledged last year there had been "an accelerated increase in child prostitution" in the country, which he blamed largely on the unofficial promotion of sex tourism in Costa Rica over the Internet.

In Nicaragua, a recent UNICEF report said, there has been significant growth in the prostitution of children between the ages of 12 and 16 in towns where taxi drivers were reported to serve as middlemen.

Several months ago, agents of the international police organization Interpol operating out of El Salvador discovered a prostitution network that was trafficking young girls from several countries in Central America to work in bars along the border of El Salvador and Guatemala. Interpol also said that it had rescued about 20 Salvadoran girls from such prostitution rings during the past three years.

While some minors are pushed into prostitution by families that are unable to support themselves, most underage prostitutes in Central America are street children, many of whom, studies show, had fled sexual abuse at home. In Honduras, the number of homeless minors has grown sharply in the aftermath of Hurricane Mitch last year.

Drug abuse, too, has become a prevalent factor in the growth of the child-sex trade. In a recent study of 300 street children in Nicaragua by the government's Family Ministry, more than 80 percent said they had started working as prostitutes over the last year, with most saying they did so to buy drugs. About a third said they needed the money to buy crack.

Standing on a corner near the Del Rey Hotel in San Jose, Juana Rojas, 14, who said she became a prostitute about nine months ago, was offering sex for $15. "A few tricks and I can buy some [crack] up the street," she said.

"I started going with men when I got hooked on crack a while ago, and since then I must have been with more than a hundred" foreigners.

Some child prostitutes offered other explanations. "I can live well, buy nice clothes and go out dancing on the nights I do not work," said Maria, 15, who shares a house here with a 14-year-old prostitute and works for a woman who sends them clients. They are paid between $50 and $200 a night.

Maria said she became a prostitute two years ago after her father committed suicide and her relationship with her mother unraveled. "Much of the time I am sad," she said. "It is hard on my self-esteem when you hear people refer to prostitutes as filthy little whores."

CAPTION: A child welfare official carries the baby of an underage prostitute who was arrested in a raid on a bordello in San Jose, Costa Rica, in July.