Start here: en.wikipedia.org/…/15_February_2003_anti-war_prot…
Specific news articles about that day:
web.archive.org/web/20040904214302/…/record.asp?r…
From Guinness World Records:
On February 15, 2003, anti-war rallies took place across the globe – the largest occurring in Rome, Italy, where a crowd of 3 million gathered to protest against the USA’s threat to invade Iraq. Police figures report that millions more demonstrated in nearly 600 cities worldwide: on the same day, 1.3 million rallied in Barcelona, Spain, 1 million participated in a peace march through the streets of London, UK, and 500,00 people in Melbourne and Sydney, Australia, joined the biggest marches since the Vietnam War peace protests.
web.archive.org/web/20100326221254/…/art.php?id=6…
The French political scientist Dominique Reynié has estimated that, between 3 January and 12 April 2003, some 36 million people took part in nearly 3,000 protests around the world against the Iraq war.
web.archive.org/web/20190921125652/…/2765215.stm
Between six and 10 million people are thought to have marched in up to 60 countries over the weekend - the largest demonstrations of their kind since the Vietnam War.
ricecake@sh.itjust.works 2 months ago
I was a bit skeptical as well, but there’s at least one seemingly reputable academic researcher who says as much: en.wikipedia.org/…/15_February_2003_anti-war_prot… (first citation).
So even if it wasn’t, one could easily be forgiven for the mistake.