Human rights campaigner Peter Tatchell is taking a leading role in protests against the state visit by Pope Benedict XVI.
He said it was inappropriate for taxpayers to foot the £12m bill for the four-day visit.
Protesters with placards were outside a number of venues in London attended by the Pope.
Mr Tatchell said the Pope's "harsh and intolerant" views were out of touch with British opinion.
The protests began early when the Pope arrived at Twickenham where he led an assembly of young people at a Catholic college.
This saw the most substantial protest of the day but some protesters were also in central London as the Pope moved on to speak in Westminster Hall and end the day celebrating Evening Prayer at Westminster Abbey.
The centrepiece of the protests is planned for Saturday with the big Pope-Nope march through London from Hyde Park Corner to Downing Street.
The protests focused on how the Roman Catholic Church had handled the child abuse scandals, the Church's attitudes to the rights of women and gay people, and the Pope's stance against condoms.
Mr Tatchell issued a challenge to the Vatican to be more open about the child abuse scandals.
He said: "We are appealing to the Pope to open up the Vatican sex abuse files and hand them over to the relevant police forces worldwide.
"The fact that the Pope is refusing to do so calls into question the sincerity of his apology.
"In fact, he hasn't apologised for his own personal failings. He always apologises for others and we think it is time the Pope himself apologised for his failure to tackle sex abuse in the church."
Derek Lennard of the Gay and Lesbian Humanist Association said: "I think the Pope in some of his pronouncements on gays, on women, the things he says about abortion are extremely hateful.
"I think that they cause great problems and destruction around the world. So, I don't think we should be paying for his visit to this country."
Another protester Ben Carey said it was the child abuse scandal that had concerned him most.
"The way the sex abuse scandal has been handled has shown the entire Church to be shockingly lacking in any sort of moral leadership.
"It has sought to protect its own, to preserve the institution rather than the very children it is there to look after," he said.
Ligia Barocot was concerned about the Pope's opposition to condoms.
"I'm not against religion at all. I think it is fine that he is here. It is just that some of the things that he preaches just aren't right for the 21st Century," she said.
Saturday's Pope-Nope march, which starts in Hyde Park at lunchtime will be followed by a rally outside Downing Street where speakers will include Mr Tatchell and the scientist and atheist Richard Dawkins.
There will also be speakers from the Survivor's Network of those Abused by Priests, Women Against Fundamentalism and the British Humanist Association.