The event, advertised on Facebook as a "Queer Kissing Flashmob" is to take place on Sunday (7 October) morning, in front of the Gothic cathedral in the city.
"The Pope is coming to Barcelona, and his Holiness is the head of an institution which has traditionally been an enemy to sexual rights for those who practice non-reproductive sex and those who don't love in church-approved ways. Consequently, we have decided to stage a protest to express our discontent," the Facebook page says.
The organisers invite participants to bring people of their own gender and to start kissing for two minutes as soon as the head of the Catholic Church enters his Popemobile and kicks off the procession to the Sagrada Familia, the Gaudi church under never-ending construction.
"No placards, no flags, no shouting and no slogans. Only kissing is allowed," the Facebook page adds. Once the kissing session is over, participants are to leave "as if nothing had happened."
The online page is back after it was temporarily shut down by Facebook's Spanish service, which considered the term "queer" to be a slur.
"Since the term was used in a self-referential manner – the only instance in which we allow slurs against protected collectives – Facebook have reinstated both the event and the page," a spokeswoman for the social network told British daily The Guardian.
Once a stronghold of Catholicism and home of the Inquisition, Spain is now one of the most liberal countries in the EU, having legalised gay marriages and adoptions for same-sex couples. Secularist premier Jose Luis Rodriguez Zapatero has also clashed several times with the Church over issues such as abortion and divorce.
It is precisely this sort of secularism that the German pope is striving to counter with his two-day visit to Spain, Vatican officials said.
Vatican spokesman Federico Lombardi said the Pope would talk about Europe in a speech on Saturday in Santiago de Compostela in northeast Spain - the third biggest pilgrimage site for Catholics after Jerusalem and Rome.
The Pope will warn against "relativism and the idea inherited from the French Revolution that in order to be fully human you have to get rid of religious tradition," said Celso Morga, undersecretary for the Congregation of the Clergy, as quoted by AFP.
Mr Zapatero will also briefly meet the pontiff at the airport, before his departure on Sunday.
Meanwhile, gay rights activists in Italy were astonished by a recent jibe by Italian Prime Minsiter Silvio Berlusconi, who in defence of his latest under-age sex scandal said: "It's better to be passionate about beautiful girls than to be gay."
Italian rights campaigner Paolo Patane said: "It's just backwards, offensive machismo that is vulgar and gratuitous."
According to Karima Keyek, a Moroccan girl who was 17 at the time Mr Berlusconi invited her to a Valentine Day's party earlier this year, where the premier displayed a marble statue of himself portrayed as Superman and organized a so-called "bunga bunga" sex party after dinner. Ms Keyek denied having sex with him, but said that she received an envelope with money and a diamond necklace from the Italian politician.