Hotel beds near Rtanj mountain, had been booked up well in advance, local tourist officials said.
"We have never had foreigners here at this time of the year," said Marina Zikic of the tourist office in Boljevac, the main town in the area.
Serbia's Mountain Rescue Services (GSS), a non-profit voluntary association, said in a statement that it had sent two teams to Rtanj just in case help was needed.
But the main reason for the sudden influx appeared to be a four-day conference organised by "The spirit of Rtanj" association, a New Age group.
Some people believe the mountain is a source of unusual electromagnetic waves and the New Age group was holding a conference on its alleged properties.
Given the conference was scheduled to end on December 23 however, the organisers did not appear to believe that the Mayan calendar predicted the end of the world for Friday.
"The newly discovered Mayan calendars indicate that there are calendars that describe and predict events on and after 21 December, so it is necessary to understand this interpretation of caution," said the "spirit of Rtanj" website (www.duhrtnja.com).
The conference would look into reports that calendars from several ancient civilisations including those the Aztecs, the Hopi Indians and the Egyptians predicted a new era beginning on Friday – on the 11th minute of the 11th hour.
Nebojsa Gajic of the hotel "Rtanj" said the district's modest quota of hotel beds – some 250 – had all been booked, with visitors from France, Germany and as far away as Australia due to arrive.
"We have some 30 per cent tourists more this year, compared to previous ones, maybe due to the 'doomsday' rumours, but also because a (New age spiritual) conference which starts" on Thursday, Gajic told AFP.
Many of the bookings had been made well in advance, but most of the expected visitors were yet to come, Gajic added.
A few latecomers, several dozen in all, had "even asked if they could park their cars here and stay there on December 21," he said.
Another hotel manager, Obrad Blecic, reported one Italian and several Austrians among this year's visitors.
"We are fully packed, quite unusual for this time of the year," said Blecic.
But Gajic warned those still keen to come: "We have no more rooms available."
Rtanj, which lies some 250 kilometres (155 miles) east of Belgrade, is normally a quiet winter resort visited mainly by hikers and climbers from Serbia. It is known for its wild countryside and fields of medical herbs.
Serbia's Tourist office (TOS) said it was aware of the sudden influx of foreign visitors.
"Our official stance is not to support such mythology, but if it is good for business, so much the better," Sandra Vlatkovic of the TOS told AFP.
A number of countries, including France and Argentina, have closed off sites which some people are convinced have some special role in what they believe is the Mayan-prophesied end of the world on Friday.