Among the protesters were actors Robyn Malcolm and Shavaughn Ruakere, who attended with her TV presenter husband Clarke Gayford.
Gayford said the bill was "alarming", had multiple flaws and would allow spies to abuse citizen's information. "What if you were hanging out at uni with someone who was a bit of an activist?"
Gayford said the turnout was heartening. "It's such a cross-section of people. There are old people, young people, wealthy-looking people. It's not a niche protest. This is something that affects everyone."
Banker David Buckleton said he joined the protest because the GCSB bill was rushed through. "There's been a good job done to stop detailed discussion," Buckleton said.
Prime Minister John Key told reporters on his visit to South Korea that the bill would give greater oversight and was "a strengthening of the previous legislation". He said Dotcom "loves the limelight".
In Wellington, more than 500 protesters marched in a crowd more than 200m long from Cuba St to the steps of Parliament. Many wore masks or held signs aloft as they chanted "we don't want your spying state" and "surveillance is for war" as they marched along Willis St and Lambton Quay.