Cher has touched down in Sydney ahead of the 40th annual Mardi Gras this weekend.

The Goddess of Pop and longtime advocate for the LGBTQI community is set to headline festivities on Saturday night.

Mardi Gras chief executive Terese Casu said Cher's arrival was exciting for this year's festival, which is made more special by the recent legalisation of same-sex marriage.

Pop icon Cher has arrived in Sydney for the Mardi Gras. (9NEWS)

"Cher has established herself as an influential, hard-hitting voice in global politics, and throughout her career has been at the forefront of LGBTQI campaigns and numerous charities," she said.

"Cher represents the same unapologetic and fearless freedom that makes the LGBTQI community so enduring and strong—positioning herself as a true gay icon.”

This weekend's glitz and glamour will be a far cry from what took place 40 years ago.

The songstress will be the headline act at this weekend's festivities. (9NEWS)

"It was something that was meant to be a fun event and it turned bloody awful," recalls Diane Minnis.

Rugged up in winter woollies, Ms Minnis was among several hundred gay and lesbian activists wandering down the city's busy Oxford Street in a late night march marking the anniversary of the Stonewall riots in New York.

With one foot in a plaster cast thanks to a motorbike accident, Ms Minnis was ambling along chatting happily with friends as they headed towards Hyde Park.

"It was quite fun to be strolling down Oxford Street," she says.

The Mrrdi Gras marchers walking the streets of Sydney in 1978. (AAP)

"Because it was winter we had on our usual daggy attire and weren't dressed up."

But the relaxed mood quickly turned to chaos when police stopped the crowd and a flat-bed truck carrying a speaker blaring out gay liberation anthems as they approached the park.

As some of the crowd began being pushed around by police, many linked arms and marched up William Street to Kings Cross where officers started brutally grabbing people and throwing them into paddy wagons.

"It was scary and very traumatic really," says Ms Minnis, who huddled in a shop doorway with a friend as police arrested 53 people.

Sydney will light up with plenty of colour this weekend. (AAP)

Ms Minnis believes if it hadn't been for the unexpected violent clash that night, there might not have been another Sydney Mardi Gras.

Since the first Mardi Gras on June 24, 1978 the march has evolved into a world-famous event held every March, when tens of thousands of people flock to Sydney to celebrate the LGBTQI community.

This Saturday, a record 2300 participants and 200 colourful floats will take part in the 40th anniversary parade, with 500,000 onlookers expected to take in all the glitz and glamour.

Among those leading the parade will be the "78ers", a group of about 250 people from that first Mardi Gras.

There will be plenty of amazing costumes on display through the night. (AAP)

"I think it will be quite emotional," says Ms Minnis.

"A lot of people are coming this year who haven't been for a long time, some not since very early on."

Back in 1978 homosexuality was illegal in NSW and those who were arrested at the first Mardi Gras had their names and occupations published in the Sydney Morning Herald.

It took until 2016 for the NSW parliament, police and SMH to apologise.

The Sydney Mardi Gras has become one of the biggest LGBTQI events in the world. (AAP)

During that long wait for an apology, Sydney's Mardi Gras got bigger and bigger.

But it hasn't been without its dramas, including the AIDS epidemic of the 1980s and a financial crisis that came close to ending the event in 2002.

Mardi Gras chief executive Terese Casu says the secret of its success lies with its activist beginnings and the creativity involved in the floats and costumes.

"I think one of the most powerful things this parade has done is activate and protest through celebration," she told AAP.

"Many of the other Pride marches around the world are protest marches and often they are with many, many, many people walking with their messages.

Thousands of people will pack into the city for this year's festivities. (AAP)

"But Sydney has taken a different and creative take on that and that's what people love. They love to come to a parade and look at that satire and to see that incredible creativity."

But while Mardi Gras has become a major event on Sydney's social calendar, many members of the LGBTQI community believe there are still challenges for their community.

Peter De Waal, another 78er, says there are still plenty of LGBTQI Australians living with the same fears of those who took part in the first Mardi Gras.

"They're still having trouble or difficulty 'if I come out, to my family, my friends, what will they say? If they want to get married they have the possibility of doing that now but what will that mean if they're employed in an organisation that has similar values to their community," Mr De Waal said.

"So these are, to my mind, really big issues and I think we as a community should try and reach out because it's awful to think there are still pockets in Australia who are living under these extraordinarily oppressive conditions."