Wearing hats, scarves and thick coats on their top halves, boarded trains at stations across the city - surrounded by bemused travellers.

Scantily-clad riders also took part in places including London, Milan, Bucharest, Berlin and Mexico City - all joined in as part of the 'celebration of silliness'.

It started off as a prank by comedy group Improv Everywhere in 2002, bemusing a few embarrassed commuters, but has since grown into a global phenomenon. 

The rules of the event, which is in its 14th year, are fairly straightforward.

Underpants must be worn and, although flashy designs are allowed, organisers prefer those involved to look - at least from the waist-up - as though they are going about their daily lives.

This is to increase the impact the participants have on the unaware bystanders.

In the inaugural event ago only seven people took part, but now thousands in major capital cities around the globe participate in what organisers call a 'celebration of silliness'.

Uniforms and business suits are encouraged to amplify the lower-half effect as are props - bicycles, prams, shopping bags or a briefcase.

Participants are also forbidden from speaking to one another and are instructed to bring 'any activity you might normally perform on the train: newspaper, book, sewing kit.' 

(dailymail.co.uk)