Hundreds of thousands of people led by 20 Roman Catholic bishops and conservative opposition leaders clogged downtown Madrid on Saturday in a demonstration against the Socialist government's bill to legalize gay marriage and permit gay couples to adopt children.

Chanting in favor of the family and children's rights, the demonstration, called by a lay Catholic group, the Spanish Forum for the Family, was held in a festive atmosphere with participants waving colorful balloons and Spanish and regional flags.

A half hour into the demonstration, organizers were claiming 1.5 million people had attended. But media eyewitnesses found the estimate difficult to believe, with most putting the crowd size at some 500,000. No police figure was immediately available.

Madrid's Cardinal Jose Antonio Maria Rouco Varela (search) was among 20 bishops at the head of the rally, along with the opposition Popular Party's leaders, Angel Acebes and Eduardo Zaplana.

Earlier Saturday, Deputy Socialist Prime Minister Maria Teresa Fernandez de la Vega (search) defended the law and accused protesters of discrimination, saying their actions meant they wanted the rights they enjoyed to be denied to others. The new law "does not oblige anyone to do anything they don't want to do," she said.

Although the protest was backed by Spain's Episcopal Conference (search) and the Popular Party, there appeared to be serious divisions over the issue within both groups. Neither the bishops' conference president, Ricardo Blazquez, nor Popular Party leader Mariano Rajoy were present.

Also noticeable by their absence were the Popular Party's leaders in Madrid — regional government president Esperanza Aguirre and city mayor Alberto Ruiz Gallardon.

The gay marriage bill is expected to become law in a matter of weeks. It has been passed by the lower chamber of Parliament and will be voted on next week by the Senate.

Opinion polls indicate a majority of Spaniards support the bill.

But demonstrators were angry at what they called the degradation of the institution of marriage and the fact that gay couples may adopt.

"Marriage can only be between man and a woman," said Agustin Cruz, 41. "It's a divine and natural law. Marriage of homosexuals is a lie. You have to call things by their name. The first lie begins when you start calling queers 'gays.' They're queers, it's not an insult, it's the definition of that race of people."

Banners reading "FamilyMan+Woman" and "A mother and father for every child" could be seen up and down the demonstration, which was attended by families and individuals of all ages. Handfuls of priests and nuns mixed with lay protesters.

Chants for Socialist Prime Minister Jose Luis Rodriguez Zapatero to resign resounded continuously.

"This demonstration is the people's response to the government's provocations," said Fr. Jose Ramon Velasco. We're not against homosexuals but allowing them to marry degrades matrimony.

"And they shouldn't have the right to adopt because if those children turn out to be homosexual, who will be to blame, the government?"

Velasco compared the bill to the beginnings of Nazi Germany in the 1930s.

"Back then the majority of people also backed Hitler just like the majority back this law," he said. "I'm serious, give it time and it will destroy the moral fiber of Spain and the West."

The Bishops' Conference last week said the gay marriage bill was the biggest challenge to the church and its values in 2,000 years.

It was the first time the church has given such a display of anti-government activism in more than 20 years.

Some 500 buses transported people to the protest from around country while special flights brought people from the Canary Islands and Spain's enclaves in Morocco.

The gay marriage bill is one of several controversial measures introduced by Prime Minister Jose Luis Rodriguez Zapatero's Socialist government since it ousted the Popular Party from office in elections in March, 2004. Others included withdrawing Spanish troops from Iraq, halting an education bill that would have made religion obligatory in schools and scrapping a national water plan that envisaged hundreds of dams and major water transfer construction projects.

The demonstration forced a complete halt to above-ground traffic in most of central Madrid.