The World Health Organisation on Wednesday cautioned against crowded indoor celebrations that could trigger a fresh surge in coronavirus infections days ahead of Easter and two weeks before Ramadan.
"In countries experiencing widespread community transmission of the virus, virtual meetings, postponing or reducing such gatherings should be seriously considered," said a WHO statement.
"Regardless of location, any religious service should be held outdoors wherever possible or be limited in size and duration, with physical distancing, ventilation, hand hygiene and mask use, as appropriate," it added.
Better would be for people to mark the festivals with those they lived with and avoid meeting others — especially if feeling unwell or in isolation or quarantine, it said.
"Indoor gatherings, even smaller ones, can be especially risky."
Experts are worried that some people may drop their guard while celebrating religious festivals, just as many countries are imposing restrictions to try to slow rising numbers of Covid-19 infections.
In the coming days, Christians celebrate Easter, while Muslims begin the fasting month of Ramadan on April 13.