Despite constant calls from organisers for social distancing, people gathered close due to the sheer immensity of the crowd.
"Scenes of thousands of protesters shoulder to shoulder makes a mockery of the Government's Covid-19 restrictions and insults every New Zealander who's followed them," Seymour said in a statement.
He said the march would be hard to watch for people who had followed the rules and lost their livelihoods.
"There will be people who've lost their jobs, lost their businesses, in some cases even lost their homes under Covid-19 restrictions. There are people trapped overseas and separated from their families while the 'lucky' ones spend 14 days all but incarcerated in hotel rooms.
"These are the law-abiding New Zealanders playing their part in the 'team of five million's' effort to beat Covid-19."
A spokesman for Prime Minister Jacinda Ardern declined to comment on the demonstrations or Floyd's death.
Police also declined to comment on whether they intended to enforce or had concerns over the crowded march.
Microbiologist Dr Siouxsie Wiles urged caution earlier today.
"Please remember that coming together in large numbers & talking loudly/shouting/singing are a perfect combination for allowing #Covid19 to spread," she said on Twitter.
"Do not go if you feel even slightly unwell. Remember #Covid19 symptoms can be very mild.
"Also, remember that people are infectious before they show any symptoms so please cover your mouth and nose with something, in case that's you."
Seymour has been calling for alert level 1, where there are no restrictions of the size of gatherings, to be put in place.
National and New Zealand First have also made the same call.
Seymour said the same rules should apply to everyone.
He expected the Government to turn a blind eye to the protest, which "should be considered a major public health risk if we took the Prime Minister's utterances seriously".
"Today's events have made a mockery of the Government and it must move quickly to Level 1 tomorrow."
