• As far as I know, the 'gathering' of Arba'een ("the 40th day"), 40 days following the Day of Ashura holds the "record" (Se below about Krumbh Mela) for largest gathering of people, though it is estimated to be #2 overall. It draws between 10 and 20 Million people, though estimates vary... Now the event and pilgrimage takes place over a span of weeks, with people arriving and departing over the duration, but I would consider the event a 'gathering' of sorts.

There have been a large number of bombings and shootings during Arba'een and they are always accompanied with accounts of the millions upon millions of people in attendance.

Some examples: http://www.presstv.ir/detail/2013/01/01/281304/muslims-heading-to-karbala-for-arbaeen/

http://original.antiwar.com/updates/2012/01/14/iraq-arbaeen-attacks-leave-71-killed-145-wounded/

List of Dates: http://www.timeanddate.com/holidays/iran/arbaeen

• The funeral of C. N. Annadurai, holds the Guinness world record for largest attendance at a single day event: ~10-15million http://en.wikipedia.org/wiki/C._N._Annadurai#Death

http://www.listedium.com/wp-content/uploads/2013/01/1e6ce2ca988e1bd21accb1a98d7997b5-orig.jpg

• Funeral of Umm Kulthum, 2-4.5 Million

• Ayatollah Khomeini Funeral 2-10 Million (estimates vary wildly): http://www.youtube.com/watch?v=2k7mpnPJWDo

• Pope John Paul II Funeral 250,000 directly during mass, 3-4 Million total: http://2.bp.blogspot.com/-xcZv1UwtdZc/TaY8RK1Ns_I/AAAAAAAACCU/SB1Fmqaujkk/s1600/arial.jpg

• Just a year ago, Tahir square held an estimated 14 million during protests, albeit over the span of a couple of weeks: http://www.newyorker.com/online/blogs/newsdesk/2013/07/tahrir-square-egypt-military-coup.html

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• And the big one: Kumbh Mela at the Ganges: ~18-30 Million in one day, with over 100 Million over its duration.

http://en.wikipedia.org/wiki/Kumbh_Mela#2013_2

http://www.youtube.com/watch?v=cQNoimABjMQ (50+ minutes)

http://www.youtube.com/watch?v=WRrIQmBwb_s (2 minutes)

This is where the world-famous pictures of people bathing in the disgusting parts of the Ganges river (sometimes next to corpses) typically come from.

Very nice blog with some beautiful pictures (Though slow-loading): http://blogs.reuters.com/india/2013/02/19/photo-gallery-allahabad-maha-kumbh-mela-2013/

Next occurrence: 2015 Kumbh Mela in Nashik (August 15 to September 13, 2015)

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• As a note, large gatherings are often poorly estimate by both the attendees and any political factions who could possibly gain anything by overstating attendance.

• Others of note:

• El Shaddai (Phillipines), Black Nazarene (Phillipines) both estimated between 4-8 Million

• Rod Stewart at Copacabana Beach 1994 (2-3.5 Million)