The black-clad protesters gave out Christmas cards to passers-by, asking them to write messages for the injured or arrested, reports the South China Morning Post newspaper.
A supporter of the event said they had collected more than 100 Christmas cards, which would be handed to Spark Alliance HK, a non-profit organisation, which will distribute it to arrested protesters.
Since the violent street clashes between protesters and police first began in June until now, more than 6,100 people, aged 11 to 84, have been arrested, according to the force.
The Hospital Authority said more than 2,600 have been admitted to hospital with injuries related to the protests, at which officers have used tear gas, rubber bullets and water cannon, and demonstrators have regularly hurled bricks, petrol bombs and other projectiles.
Ally Lai, a second-year Chinese University student, said she hoped to write to those arrested, including some of her classmates, because "they have sacrificed a lot for our freedom and the next generations".
A Japanese woman living in the city said she felt "saddened" by the huge numbers of arrested people and hoped that, by writing them a Christmas card, she could let them know they still had support and should not give up.
Monday's protests remained peaceful with a minimal police presence, as participants were not seen blocking roads.
In the city's Central area, hundreds of protesters, mainly office workers, gathered in a mall, responding to online calls for a flash mob against alleged police brutality.
( With inputs from IANS )