Tens of thousands of people gathered in Melbourne's CBD for an 'Invasion Day' rally on Thursday.

The crowd at Federation Square - estimated by some to be up to 50,000 people - is believed to have eclipsed the number of people who attended the earlier Australia Day march along Swanston Street.

Protesters gathered at Parliament House on Spring Street about 11am, just as the Australia Day parade was beginning nearby.

The tune of Waltzing Matilda could be heard drifting from the bagpipes on Swanston Street as 'Invasion Day' protesters gave speeches in front of Parliament House.