Public masses also resume throughout the country after a two-month hiatus, while restaurants, bars, cafes, shops and hairdressers, among other businesses, are all expected to reopen.
In the face of much opposition, including from Pope Francis, churches in Rome were closed at the beginning of the coronavirus emergency in early March.
Most, however, opened shortly thereafter, with entry reserved for prayer only.
"I share the joy of those communities who can finally reunite as liturgical assemblies, a sign of hope for all society," the Pope said during his live-streamed prayer yesterday.
Italy's lockdown not only extinguished most business activity in the country, but radically disrupted Italians' personal lives, including attending mass.
Pope Francis has been livestreaming mass from a chapel at his residence inside the Vatican City.
The pontiff is not yet expected to lead any public religious ceremonies either in the basilica, which can accommodate 60,000 people, or in St Peter's Square, as the Vatican seeks to avoid crowds.
The Pope will, however, celebrate a private mass today, broadcast by video, in front of the tomb of John Paul II, on the 100th anniversary of the his birth.
On Friday, the cathedral said it had introduced gadgets worn around the neck that beep softly, flash and vibrate if visitors approach too closely to one another.
In preparation for the reopening of St Peter's, the church was disinfected on Friday, with workers spraying down the surface of the 23,000-square metre site.
The Vatican has applied the same anti-virus measures as Italy, where the official death toll from the virus stands at nearly 32,000.
The basilica, as well as three other papal basilicas, is expected to follow a recommendation from Italy's interior ministry limiting attendance at religious celebrations in enclosed places of worship to 200 people.
Across Italy's tens of thousands of churches, Catholics will be able to attend not only masses but also weddings and funerals, provided they abide by a series of measures, including wearing masks and sitting or standing well spaced apart.
More than 800,000 commercial activities which have been under lockdown since Prime Minister Giuseppe Conte imposed restrictions on 9 March should be able to reopen today, said Confcommercio, Italy's largest business association.
https://www.rte.ie/news/world/2020/0518/1139000-st-peters-basilica/
