A network of religious orders against human trafficking says this summer's World Cup in Brazil will encourage an increase in child prostitution.
Sister Gabriella Bottani, an Italian nun, said the "greatest concern is linked to the increase in the exploitation of child prostitution".
Bottani said the risk of child exploitation increased by up to 40% during the World Cups in Germany in 2006 and South Africa in 2010, and would be likely to rise again during Brazil's month-long tournament starting in June.
The campaign, "Play For Life, Report Trafficking", will involve nuns and others handing out leaflets at airports and key tourist areas in Brazil encouraging people to report suspected child prostitution or enslavement to police, according to an Associated Press report.
"Without awareness, without acting together in favour of human dignity, the World Cup finals may turn out to be a terrible shame instead of a feast for humanity," said Sister Carmen Sammut, president of the International Union of Superiors General, another of the member organisations.
Of particular concern are gangs recruiting under-age sex workers. Around 500,000 minors, many from poor rural areas sell their bodies for sex, cigarettes or to buy food.
The girls' parents or relatives act as pimps on a notorious road for child abuse, the BR-116, a 2,800-mile highway that runs the length of the country.
Posters at airports warn tourists that they face prison if caught soliciting minors. However, the police and courts regularly fail to follow through with action, say child prostitution activists.
A study by a parliamentary commission in Sao Paulo reported last week that there was an increase in child prostitution and sex abuse around the Corinthians stadium where the World Cup opening ceremony will be held on 12 June.
Other investigations claim that child prostitutes are positioned by pimps around other World Cup stadiums, including the Maracana in Rio.
"There is a real culture of silence," said Matt Roper, who has set up a charity called Menina Danca to help girls in the town of Medina, just off the BR-116.
The town of Medina has a population of just 15,000 but is notorious for drug gang murders and child abuse. Social workers believe about 100 girls sell their bodies there.
"Mothers or family members have no problem taking their daughters to a roadside brothel," Roper told the Times.
"It's seen as completely normal."