I was actually part of that. They had synchronized protests in countless cities across the globe.
It was a no-brainer. The war reeked, and the manufactured consent on the cable news and other major media channels was palpable. The WMD argument was sufficiently debunked before the war started and the pivot to "We're doing it to bring freedom to the Iraqi people" is so ridiculous I was lucky I wasn't drinking milk the first time I heard it on the "news."
It would have been a little harder to sell the truth...-that we wanted to secure Iraq's oil supply and free more of it up to the open market to make sure our gas bills are cheaper, and also establish a stronger military presence in the region to interact with Iran if need be. When you put it like that, it's a lot harder to justify spending trillions of dollars and killing hundreds of thousands of people. We even fucked that up, and removed the counterweight to Iran in the region, escalating their influence in the region beyond their wildest dreams.
On a side note, people and speakers at anti-war rallies are embarrassing. Their reasoning and rhetoric are just awful, -at least the speakers I heard. There were a lot of good reasons to oppose that war...didn't hear any of them articulated at the protest.