On his decision to join the protest, King said he felt it was important to be among "real people".
During his speech he urged protesters to remain peaceful and non-violent.
King has often spoken out against the vaccine mandate, and National's support of it.
He told the Herald last week: "For me, mandates are a line in the sand because it's my fundamental right … to choose what goes in my body. I don't accept the 'greater good' argument.
"I said to the National Party guys, 'I can't support that. This is my position I can't go against'. I'm still hoping they will change their tack, regardless of what happens to me. [If they don't] they'll be on the wrong side of history."
King has often spoken out against the vaccine mandate. Photo / Michael Neilson
King was a one-term MP in Northland, elected in 2017 then losing to Willow-Jean Prime by 163 votes in 2020.
He has acknowledged his comments have likely killed any chance of him getting back into Parliament.
Rain has fallen in buckets as protesters continued to flood onto Parliament grounds.
Along with trenches dug to drain the pooling water, carpets were laid, as well as straw in an attempt to stop the Parliament lawn turning into a swamp.
Despite the rain increasing in intensity, protesters continued to arrive from all across the country, with trucks and vans and campervans blocking even more streets through the CBD. Many are bringing with them ample supplies ranging from food and medical services to sanitary items and even nappies.
Protesters were seen passing the person on a stretcher with a blue tarpaulin covering them to police over barricades.
The person was been taken to Wellington Hospital by ambulance and is currently in a moderate condition, a Wellington Free Ambulance spokeswoman said.
It follows a protester suffering a medical event on Friday night. Because of the hundreds of vehicles blocking the roads surrounding Parliament, ambulances could not get to him.
