Pope Benedict XVI faced Saturday the first major protests of his visit to Britain as police kept six street cleaners in custody for questioning over a suspected "terrorist" plot linked to his trip. The 83-year-old pontiff was set to lead a huge open-air prayer vigil in Hyde Park and meet with Prime Minister David Cameron as he pushed ahead with his busy itinerary despite Friday's security alert.But a coalition including victims of abuse by Catholic priests, atheists, pro-abortionists, demonstrators calling for women priests and protesters angry at the cost of the visit are set to rally ahead of the vigil.Organisers said they expected around 2,000 people to turn up at the "Protest the Pope" event, gathering nearby Hyde Park and then marching through central London for a rally outside Downing Street. It will be the first serious rally against the head of the Roman Catholic church on the four-day trip -- the first ever papal state visit to Britain by royal invitation -- with only minor protests having taken place so far.Security will remain tight for Benedict on Saturday, with police closing key roads in the capital along the route his popemobile will take, although police said the arrests had not changed their existing plans for the visit.Click image to see more photos of the pope's visit to the U.K.In addition to his prayer vigil, at which tens of thousands of people are expected, he is also to attend a service at the Roman Catholic Westminster Cathedral and address thousands of young people in the piazza outside. The Vatican insisted the pope was "calm" after the arrests.Counter-terrorism police in London swooped at dawn on Friday to detain five men, aged between 26 and 50, "on suspicion of the commission, preparation or instigation of acts of terrorism", Scotland Yard said.A sixth man was held later Friday. Reports said the suspects were North African and that they were detained on a tip-off, but that the arrests were initially precautionary because they had little information to go on.Westminster council confirmed the first five men arrested worked for an environmental services company hired by the council to clean the streets The arrests came shortly after the head of MI5 warned in a speech of increased threats to Britain from Somalia and Yemen as well as from extremists opposed to Northern Ireland's peace process.Benedict pressed on unfazed on Friday with his schedule, which included a highly public show of unity with Archbishop of Canterbury Rowan Williams at Westminster Abbey.He also gave a speech Friday at the Houses of Parliament to an audience including four former prime ministers, warning about the "marginalisation of religion" in largely secular Britain and further afield.Media on Saturday said the pope's visit had gone well so far despite a run-up clouded by the paedophile priest scandal rocking the church and by a Vatican aide's comments comparing Britain to a "Third World Country."Many newspapers hailed him for his comments on religion, with the Times saying it was "turning into a remarkable success as the rift with Rome narrows" and others praising his remarks defending the celebration of Christmas.Benedict is only the second pope -- after John Paul II -- to visit predominantly Anglican Britain since Henry VIII split with the Roman Catholic Church in 1534 over its failure to annul his marriage.