People in Sydney and Melbourne have joined mourners around the world at vigils to honour the victims of a mass shooting at a gay nightclub in Orlando during the country's LGBTI Pride Month.

Key points: Almost 50 killed in mass shooting at gay nightclub in Orlando, Florida

Almost 50 killed in mass shooting at gay nightclub in Orlando, Florida Shooting comes during LGBTI Pride Month in US

Shooting comes during LGBTI Pride Month in US Australians join vigils in solidarity with LGBTI community

Almost 50 people were killed and another 53 injured after a heavily-armed gunman opened fire at Pulse nightclub in Orlando, Florida — the worst mass shooting in modern US history.

On Monday night the Sydney Harbour Bridge was lit up in rainbow colours, in a show of solidarity that follows similar moves by iconic buildings around the world.

About 400 people turned out to a candlelight vigil in Newtown in Sydney where people spoke about the shock of the homophobic attack.

"It was a hate crime, and it was due to a lack of understanding and the way that I take it is, if we have knowledge within our community and we teach people and educate people on what is OK and isn't OK, things like this wouldn't occur," attendee Erin Young said.

Chris Pycroft of the NSW Gay and Lesbian Rights Lobby said Australia's LGBTI community had taken the attack very personally.

"Particularly when you go to a venue like a gay club you expect that to be one of the places where you feel safe and supported," he said.

Candles placed during vigil at Newtown. ( ABC News: Sarah Hawke )

A rally in the inner-Melbourne suburb of Fitzroy attracted about 500 people and many bowed their heads for 50 seconds of silence — one second for each victim.

"We are not a community that will go away," OJ Blain-Allen told the crowd, urging LGBTI people to stand together.

"We will not hide, we will not be intimidated by this horrible event."

Sorry, this video has expired Sydney lights iconic structures in a show of solidarity

Buildings in Melbourne, including the Arts Centre and National Gallery of Victoria, were lit up in rainbow colours.

The Eiffel Tower in Paris will be lit up in the American colours of red, white and blue, while the One World Trade Centre in New York displayed the colours of the rainbow.

The lights of the Empire State Building remained dark to honour those killed.

Melbourne Town Hall lights up in rainbow colours in the wake of the Orlando shootings. ( Twitter: City of Melbourne )

In Florida, Jeff Prystajko, the director of communications at Orlando-based non-profit group Come Out With Pride, told the ABC everyone in the local gay community would be affected by the shooting.

"We are such a tight-knit community. We are all going to know people who knew people who were victims here. For everyone here it was a very tragic and senseless day. Wish we could turn it back," he said.

He said he had been to Pulse in the past.

"Maybe there was a bartender there or someone who I was standing next to or on the dance floor with; I'm sure some of those people are not with us today. That's absolutely terrible to think about," he said.

"We know we are going to come out of this stronger.

"We are dealing with this in terms of trying to come together and make sure we are all staying together and staying strong through this," he added.

People at Darwin's Civic Park pay homage to those affected by the Orlando shootings. ( ABC News: Jacquelin Breen )

In June the US celebrates LGBTI Pride Month in commemoration of the Stonewall riots, which were a tipping point for the country's Gay Liberation Movement.

Large-scale public events, including parades, picnics, concerts, and workshops, are held across the country and attract participants from around the world.

Mr Prystajko said the celebrations would go ahead despite the shooting.

"We know that this year is going to be certainly different from every other Pride that has been here in Orlando in the past," he said.

"But we will make sure people come together, they understand what the circumstances are, and know what we are fighting for. This is a situation where we are not going to back down."