On the eve of the 84-year-old pontiff's arrival in the Spanish capital to celebrate lavish World Youth Day events, more than 100 groups opposed to the visit prepared to march Wednesday evening.
The joint protest unites many causes, including groups seeking a change in the Church's attitude to gay rights and those fighting for a clearer separation of Church and state.
There is even a separate plan for a gay and lesbian "kiss-in" once the pope gets here Thursday.
But the outcry that has struck a chord with many -- including some priests -- is over the official 50.5-million-euro ($73-million) price tag, excluding the cost of police and security, of the Madrid celebrations.
The protest groups, some of which argue the real cost of the event to taxpayers is more than 100 million euros, are joining under the slogan: "The pope's visit, not with my taxes."
Organisers say most of the cost will be covered by a registration fee from the pilgrims, and the celebration will be a massive tourist boost for Spain.
But for many the celebration is jarring at time when the economy is faltering, the government is making painful cuts and unemployment is at 20.89 percent. For those under 25, the jobless rate is over 45 percent.
"We criticise this scandalous show at a time of such a terribly distressing economic situation, with entire families unemployed," said Evaristo Villar, of Redes Cristianos (Christian Networks).
"This ostentation is causing a lot of damage and distancing a lot of people" from the Church, he said.
Many of those in Spain's 15-M "indignant" movement -- launched on May 15 against the management of the economic crisis -- are also taking part in the protest.
"Just months after visits to Santiago and Barcelona, Mr. Ratzinger -- Benedict XVI to the Catholics -- persistent in his idea of 'reconquering' a country that he sees as veering away from its moral and religious programme, returns to Madrid," the protesters said in a statement to be read out at the rally.
It appealed to all citizens, "regardless of their personal convictions," to stop authorities from granting the Church "privileges that belong to past eras and an undemocratic heritage."
Spanish gays and lesbians say they will hold a separate protest "kiss-in" after the pope's arrival Thursday.
The kiss-in, being organised on Facebook, is to protest against the Church's "moral condemnations of sexuality," said Jaime del Val, a spokesman for one of the gay groups involved, Asamblea Transmaricabollo de Sol.
When the pope last visited Spain in November last year, he was confronted by a homosexual kiss-in in Barcelona.
About 200 gay men and women couples locked lips to demand the Church recognise their right to be gay as he paraded through Barcelona's streets November 7 in his transparent "Popemobile".
For the August 16-21 Catholic celebrations, traffic has been barred in much of central Madrid and a huge white stage has been erected for events in the emblematic Cibeles Square.
Huge speakers blare out pop music through the day as hundreds of thousands of fans in floppy hats swelter in the August heat.
The Church has opened 200 white confessionals in the form of boat sails along the main thoroughfare through Madrid's Retiro park.
The pope will hold a "Prayer Vigil" on Saturday evening at an airbase southwest of the capital, where the pilgrims will spend the night on an esplanade the size of 48 football pitches.
Benedict celebrates mass there on Sunday morning at a white altar almost 200 metres (660 feet) long in front of a wave-shaped stage and under a giant parasol "tree", made of interwoven golden rods.
A secular protest group, Europea Laica, said it hoped several thousand protesters would join the march.
"It is in defence of the construction of a secular state, the separation of the Church and state, and against the financing of the Churches by the state," said Europa Laica president Francisco Delgado.
One Roman Catholic group called Hazte Oir (Make Yourself Heard), called on the authorities to ban the protest march, with a petition describing it as "an expression of intolerance and religious hatred".
Why would they host such a thing, with taxpayer money, no less? Especially under current economic conditions? Unemployment is at 20.89% in Spain (according to the article), and this is what they're wasting taxpayer money on? Really?
An official papal visit is one thing, but no need to make it a $73 million extravaganza.
Good for the protesters mentioned in the article; hopefully they'll help prevent something like this from happening again.