WHO warned that despite some “fragile progress” coronavirus transmission remains “widespread and intense” across Europe.
WHO said indoor gatherings were particularly risky because they were likely to bring together groups of people from different age categories and the different households may not be following the same prevention measures.
“Gatherings should be held outside if possible, and participants should wear masks and maintain physical distancing,” said WHO in guidance that was issued on Wednesday.
“If held indoors, limiting group size and ensuring good ventilation to reduce exposure risk are key.
“It may feel awkward to wear masks and practise physical distancing when around friends and family, but doing so contributes significantly to ensuring that everyone remains safe and healthy.”
WHO also advised that any religious celebrations or holiday markets take place outdoors, be limited in size and duration with social distancing, ventilation and mask use.
Scotland and Wales appeared to deviate from a UK-wide approach for the holiday period which would allow three households to mix over the Christmas period from December 23 to 27.
In Wales, people are being advised to limit their bubbles to two households, while in Scotland the guidance on what people should do is being strengthened.
Boris Johnson urges extreme caution as restrictions ease at Christmas
The Welsh First Minister Mark Drakeford, said a joint four-nations statement was still expected, but would not say whether the two-household guidance would apply across the UK.
Scotland’s First Minister Nicola Sturgeon recommended that those people forming a Christmas bubble should only meet up on one day and not stay overnight “unless it is unavoidable”.
She said: “Firstly and unequivocally the safest way to spend Christmas this year for you and for those you love is to stay within your own household and your own home.
“My strong recommendation is this is what you should do if at all possible.”
In Northern Ireland, First Minister Arlene Foster said the public must take “all and every precaution” at Christmas and proposals for further restrictions will be brought forward on Thursday.