“We're protecting a centuries-old institution, we will not stand for interference from either the Church or the government,” Paolo Jorio, director of the San Gennaro museum where the jewels are kept, told AFP.
The protest was sparked by a decree issued by Interior Minister Angelino Alfano.
Critics say his move opens the door to Church control as it threatens a lay council that for centuries has guarded the jewel-encrusted treasures, donated by kings and aristocrats in honour of San Gennaro.
Over three thousand locals, some wearing T-shirts with pictures of the saint, tied white handkerchiefs to the gate of the museum and neighbouring chapel, with many holding signs reading “Don't touch the treasure”.
The lay council was established in the 16th century as the southern Italian city struggled to overcome a series of devastating misfortunes: a resurgence of the plague, a siege by the French and an eruption by the volcano Vesuvius which set off earthquakes.
Those who survived pledged in 1527 to build a chapel to their patron saint—known as St Januarius in English—who was beheaded in 305 AD during the persecution of Christians by the Emperor Diocletian.
Not only was the chapel built with the city's money, it was presided over by the newly-formed council, made up of 12 lay citizens and the mayor, and came to house one of the world's most important collections of religious treasures.
‘Diamonds, rubies, emeralds'
As well as silver busts of saints, there are heavily-jewelled necklaces and earrings and a golden mitre, the ceremonial headdress of bishops, which is studded with 3,326 diamonds, 164 rubies and nearly 200 emeralds.
Alfano ruled that the council is the same as any other caretaker body which manages religious buildings—such as Saint Peter's Basilica in Rome—and ordered that four of the committee's posts should be in the hands of the Church.
But the council and its supporters say it is much more than that, particularly because it oversees the Gennaro miracle.
Three times a year a ceremony is held in which locals pray for miraculous liquefaction of the saint's blood, held in a glass vial clutched by a priest or cardinal.
If the blood does not return to liquid from its coagulated state, it is seen as a bad omen for the city—a harbinger, say the superstitious, of a disaster, perhaps even the eruption of mighty Vesuvius, which looms over the city.
A white handkerchief is waved to announce a miracle.
It is an important tradition, both for devotees and the Church, but it is the council which safeguards the vial, and the mayor who invites the Archbishop of Naples into the chapel for the liquefaction ceremony.
Critics say the Church has tried several times down the centuries to get control of the vial and treasure.
“We think Cardinal Crescenzio Sepe (Archbishop of Naples) has applied great pressure on Alfano, in order to extend his influence over one of the most well-known symbols of popular religion,” Jorio said.
He said the council would appeal the decree in court and further protests would be held in a bid to get it reversed.
Naples' mayor Luigi De Magistris, head of the council, said they would do what it takes to “makes sure what San Gennaro gave us is not diminished".