Thousands of Australians have marked the country's national holiday with rallies in support of the nation's Indigenous people, many of whom describe the anniversary of the day a British fleet sailed into Sydney Harbour as "Invasion Day".

In Sydney, the capital of New South Wales — Australia's most populous state — social media showed a large crowd gathered at an "Invasion Day" rally on Thursday in the central business district, where some people carried Aboriginal flags and an Indigenous smoking ceremony took place.

Similar actions were scheduled in other Australian state capitals, including in South Australia's Adelaide, where around 2,000 people attended, according to the Australian Broadcasting Corporation.

Indigenous activist Paul Silva, speaking to a crowd of thousands in central Sydney, said the national holiday should be abolished. 

"They invaded our lands, killing our extended families, turning our warriors into slaves," he told the crowd. 

"How can this day be celebrated?" 

Indigenous poet Lizzie Jarrett said Sydney was "ground zero for a genocide of First Nations people". 

"You think we're angry? Wouldn't you be angry?" she asked the crowd.

Speaking at a flag-raising and citizenship ceremony in Australia's capital, Canberra, Prime Minister Anthony Albanese honoured the nation's Indigenous people, who have occupied the land for at least 65,000 years.

"Let us all recognise the unique privilege that we have to share this continent with the world's oldest continuous culture," Albanese said.

Argument rages over how history should remember a fleet of 11 British ships carrying a human cargo of convicts arriving at Port Jackson in present-day Sydney on January 26, 1788, to start a penal colony, viewing the land as unoccupied, despite encountering settlements.

Australian historian Lyndall Ryan has estimated that more than 10,000 Indigenous people were killed in 400 separate massacres since British colonisation first began. 

READ MORE:Thousands march for aboriginal rights as Australia marks national day

'Entrenched inequality'

An annual poll by market research company Roy Morgan released this week showed nearly two-thirds of Australians say January 26 should be considered "Australia Day", largely unchanged from a year ago. The rest believe it should be "Invasion Day".

Amid the debate, some companies have adopted flexibility around the observance of the holiday.

Australia's largest telecoms company, Telstra Corp Ltd, this year gave its staff the option to work on January 26 and take another day off instead.

Many of Australia's 880,000 or so Indigenous people lag behind the country's 25 million citizens on economic and social indicators in what the government calls "entrenched inequality".

This year's holiday comes as Albanese's centre-left Labor Party government plans a referendum on recognising Indigenous people in the constitution and requiring consultation with them on decisions that affect their lives.

The constitution, which came into effect in January 1901 and can't be amended without a referendum, does not refer to the country's Indigenous people.

READ MORE:Australian PM Albanese outlines draft Indigenous recognition vote