The flash-mob pillow fights took place in 115 cities, according to Britain's Daily Mail, with varying degrees of fun and ferocity.
Many NYC pillow fighters turned out in rabbit costumes for the feathery pre-Easter event in Washington Square Park, the New York Daily News reported.
While in Shanghai, police officers cracked down on a small group in the city's People's Square after about five minutes.
The fights are organised as part of the Urban Playground Movement via Facebook and also on www.pillowfightday.com -- which features a list of participating cities and a guide to pillow fight etiquette.
The website offers no explanation as to why people might wish to take part.
Taiwanese student Jiang Shin-Yu, 16, who joined around 100 others to fight in the capital, said it was a great way to relieve stress.
"I am so happy that I finally got to release all my stress today," Sky News quoted Shin-Yu as saying. "There is a lot of pressure on me from my academic work, problems in my relationship and arguments with friends."
The site discourages pillow fights in parks -- not only due to litter concerns but because apparently "most events in parks are boring."
Hundreds took part in a pillow fight in front of Berlin's famous Brandenburg Gate, and others in front of the National Gallery at Trafalgar Square, London.
The record for the largest pillow fight was 3,706 people at a BBC Children in Need event in Minehead, Somerset in 2008, the Independent wrote.