Protesters have taken to the streets in Sydney and Melbourne today to march against vaccinations and tennis star Novak Djokovic's second visa cancellation.

It comes at the same time that the country reaches almost 20 million people fully vaccinated against Covid-19.

Police in Sydney closed some parts of the CBD as thousands marched against COVID-19 vaccinations for children.

The crowd snaked from Prince Alfred Park to Hyde Park.

READ MORE: The reasons why Novak Djokovic's visa was cancelled a second time

Djokovic supporters were amongst them.

The men's tennis world No.1 was taken into immigration detention in Melbourne today ahead of his appeal against the second cancellation of his visa to be heard in court at 9.30am on Sunday.

In Melbourne, protesters galvanised to show their solidarity with the Serbian sportsman.

The march was titled "Oppose Australian Open madness", with protesters demanding that Djokovic be allowed to play, COVID-19 mandates be stopped, and "medical segregation" end.

READ MORE: Waves hit Tonga amid tsunami warning following huge volcanic eruption

It comes after Immigration Minister Alex Hawke described Djokovic as a "talisman of anti-vaccination sentiment" and a risk to civil unrest.

Almost 20 million Australians have now received at least two doses of a COVID-19 vaccine, almost 80 per cent of the population.

Almost 5 million people over the age of 18 have received a third COVID-19 vaccine dose.