A crowd of more than 2,000 people protesting against the downgrading of Gerard Baden-Clay's conviction for murdering his wife Allison has heard calls for stiffer penalties for crimes of domestic violence.

Hot and humid conditions did not stop people gathering in King George Square in Brisbane's CBD to honour the memory of Allison, who was killed in 2012.

They were protesting against the recent decision by Queensland's Court of Appeal to set aside Gerard Baden-Clay's murder conviction and downgrade it to manslaughter.

Many supporters were wearing yellow, a favourite colour of Allison.

Australian senator Glenn Lazarus, who spoke at the rally, used his time to call for changes to the law.

"When the people, the law makers, the law changers ... when they see the response that the community has come here with today they will have a real serious look and hopefully get some stiffer penalties involved with domestic violence," he told the crowd.

Child safety advocate Bruce Morcombe, whose son Daniel was murdered in 2003, told the crowd fundamental change was needed.

"We expect the judicial system to move with the times, they have to, because we all need to maintain the genuine belief that our legal system works," he said.

"Equally, our laws must move with the times, community attitudes change, then the laws must change."

Mr Morcombe said a possible counter-appeal by the DPP must go ahead.

"We acknowledge the daily struggle that her parents, extended family and her children endure," he said.

"We call on the Queensland Attorney-General to correct this imbalance and provide some certainty for the future, so that a punishment meets the community expectations. This is not right."

Sorry, this video has expired Nicole Morrison speaks at a rally against Gerard Baden-Clay's conviction downgrade

Rally organiser Nicole Morrison, who was a friend of Mrs Baden-Clay, told the crowd people across Queensland were disappointed with the judiciary.

"The shock and confusion of last Tuesday's decision resonated within not only those involved with Allison's case but also the community at large, and this is the reason we are all standing here today," she said.

Marilyn Wybrow said it was the first time she had taken part in a rally.

"I've never protested anything in my life before but I felt so strongly about this — that it's wrong," she said.

Marie Morris said she travelled from the Gold Coast to attend the rally.

Allison Baden-Clay was reported missing by her husband Gerard in April 2012. ( Supplied )

"I felt I had to come, for Allison's parents and her children," she said.

Bob Probert said the appeal decision was "an insult to those 12 members of the jury".

Phyllis and Richard Streeton, who have never protested before, said they were angry at the court's decision to downgrade Baden-Clay's murder charge to manslaughter.

Ms Streeton said the jury found had found Baden-Clay guilty and that it was an injustice to have the downgrade the conviction.

"To think that a jury have convicted him and then you have a legal team come in and say, 'well there's all these issues on the side' ... it's got to stop," Ms Streeton said.

Mr Streeton said he was angry about the appeal decision.

"The injustice of the whole thing - it's a shambles," Mr Streeton said.

Kelli Schuh said she felt so strongly about the court's decision that she took her three young children to the rally.

"My youngest is only eight - and they understand that it's wrong," she said.

Lesley Stafford said she attended to show her support for the three Baden-Clay children.

"This is my first rally for this sort of thing and only because I feel so emotional about it, for everybody's young children," he said.

Ms Morrison said the rally was a "very peaceful, positive collection of people within the community".

"To stand together just to let people know — whether they knew Allison or not — have been very affected by her death, by her lengthy trial, and then following the guilty conviction [of] murder, the downgrade in those charges," she said.

"I think the rally ... will be just a really positive message that as a community, we are confused by the downgrading of the [conviction], but we're standing together to start this conversation to hopefully use it as a platform for some positive change."

Queensland's Chief Justice Catherine Holmes and two other Court of Appeal judges last week set aside the murder conviction, replacing it with manslaughter, saying there was not enough evidence to prove the former real estate agent had intentionally killed Allison.

People have also shared photos on social media using the hashtag #doingit4allison.