I've picked the most interesting selection I could find from the worlds largest gatherings. The largest anti war protest was in Rome in 2003, against the Iraq war, where more people gathered than for the annual pilgrimage to Mecca, the Haj. In fact, Sao Paulo's Gay Pride festival is roughly the same size as the Haj, with 2.5 million attending in 2006 and the Haj isn't even necessarily the largest gathering in Islam, with 14 million people attending the Shiite Arab'een in Iraq. Religious ceremonies dominate, with a Papal mass in Ireland in 1979 resulting in a third of the population (1.25M) gathering in a single field, but paling in comparison with religious gatherings in India, including the Kumbh Mela which drew an unbelievable 60-70 million people over 45 days in 2007, making it the largest gathering in history.