Every June for centuries, Pisa and its leaning tower have been illuminated by thousands of candles and oil lamps to celebrate its patron saint, Ranieri, in one of the most atmospheric festivals in Tuscany.

But tomorrow night, for the first time, participation in the Luminara festival will be compulsory under a new city law, with police set to roam the twinkling city to slap fines of up to €500 [£420] on anyone caught without a candle in their window.

The city council insists the new law is essential to prevent "black holes" of darkness ruining the overall effect of the 80,000-candle festival.

"Unfortunately black holes in the buildings along the river are a real offence to the city's beauty," Federico Eligi, the council's celebrations chief, told the Corriere della Sera newspaper. "There's also a security issue, since the public lighting system is completely shut off and the candles are the only things illuminating the streets."

But some Pisans are furious, especially since the law requires non-Christians to take part in the Luminara. "You can't require people to celebrate a saint with a candle, and you can't deny people the freedom not to participate in a festival," said Alfonso Maurizio Iacono, head of literature and philosophy at Pisa University.

Remo Bodei, another philosophy professor, disagreed: "St Ranieri is no longer a religious festival but a celebration of the city, and an ugly Luminara isn't decorous. Now it will be even more charming."