The phenomena you remember are called "flash mobs".
"In modern usage, flash mob describes a group of people who assemble
suddenly in a public place, do something unusual for a brief period of
time, and then quickly disperse. They are usually organized with the
help of the Internet or other digital communications networks.
The term has also been applied to distributed mobs, who use similar
means to coordinate sudden large scale simultaneous actions in
multiple locations. An example of such an action is the widespread use
of mobile phones in the 2005 civil unrest in France to coordinate
widespread social disruption."
Examples of flash mobs throughout the world:
The first flash mob. An interview with Bill Wasik, where he calls it a
Some examples of flash mobs:
E-Mail Mob Takes Manhattan
Brazilian flash mob has hit the busiest road in Sao Paulo.
"Around 100 people gathered in Avenida Paulista and pointed remote
controls at a giant screen, as if they were trying to change channels.
After exactly three minutes they put the controls away and walked off
as if nothing had happened, Folha de Sao Paulo newspaper reports.
The first Brazilian flash mob happened last week when a crowd
converged on a Sao Paulo street corner, removed one shoe each and beat
it on the pavement several times.
The flash mob phenomenon, in which crowds organised by email and
websites perform pointless stunts, started in the US and has spread
around the world."
"On 23 April 2006 in Manhattan, New York City, about 80 people arrived
in Union Square North at 3:30pm wearing blue short sleeved polo
shirts, khaki pants, a belt, and black dress shoes, very similar to
the uniforms worn by the employees of Best Buy. They proceeded to
enter the Best Buy on 23rd Street at 15-second intervals. While in the
store they would each stand at the end of the aisles in different
departments; they didn't claim to be employees of Best Buy, but agreed
they would be friendly, helpful, and answer questions from any
customers if they were approached. This flash mob was confusing to
employees as well as customers. One employee claimed it was just like
the The Thomas Crown Affair. This flash mob was organised by Improv
Flash Mob Description
"There was also a smart mob this afternoon. The idea is, a bunch of
people get organized via email or whatever, and at the last minute,
they decide where to go. They all show up at the exact same time, do
something meaningless for ten minutes or so, and then all vanish at
once. Sounds like fun. We met up at the magazine rack of a 7-11 in
town, about twenty or thirty maybe, and it was all well and good.
Could have been a better location, but it's nice to see that many
people will do something like that just for fun."
Flash mobs in Paris (in french)
Video from Rome Flash Mobs
Brisbane Flashmob : PastEvents
Flash mobs in Sydney:
Click on the mobs in the left hand menu to see past events.
Some sites that may interest you:
"show up at the exact same time"
"flash mobs" examples
I hope the information provided is helpful. If you have any questions
regarding my answer please don't hesitate to ask.