What was supposed to be a free-spirited Sunday afternoon Gautrain ride turned out to be a trip to the cop shop for 34 people when they dropped their pants on the Gautrain yesterday.

Kuba Granicki, who was among the almost 150 people who took a ride from the Rhodesfield station, in Kempton Park, on the East Rand, to Sandton, northern Johannesburg, clad only in their underwear, said the group was detained for about three hours at the Kempton Park police station.

"First they wanted to charge us with public indecency, but there was no indecency; then they wanted to charge us with an illegal gathering in a public space."

The group, part of a Facebook effort to stage the "first annual no-pants Gautrain ride", got onto the 3.30pm train at Rhodesfield yesterday, dropped their pants a minute after the doors closed and rode in their frillies to Sandton.

Organiser Ben Pienaar said the group got out at Sandton, took the stairs to the other side of the station and took the train back to Rhodesfield.

On their arrival, they were met by the police.

"What should have been a fun afternoon turned out to be a really nasty incident," said Granicki.

"This country has lost its sense of humour."

Pienaar said the stunt was in celebration of a similar event staged on the New York subway, which marked its 10th anniversary yesterday.

"The event is held in 16 countries in 48 cities."

Pienaar said none of the participants, men and women of all ages and races, behaved indecently.

"Everybody had something like boxer shorts under their clothes," he said.

The group was released just after 6.30pm after being fined R350 each for "breaking a Gautrain rule".

Gautrain's Barbara Jensen said icily: ''There were other people on the train and it is actually not [a venue for] a bachelors' party.''