Lyle Howe, a black lawyer with a high public profile in the city, was found guilty last month by a jury in the Nova Scotia Supreme Court of sexually assaulting a 19-year-old woman on March 20, 2011.
Since the verdict, more than 6,000 people have become members of a Facebook page campaigning on behalf of Howe.
David Sparks, a community organizer, said the purpose of the rally was to show the number of people who believe Howe is innocent.
"It's to show Lyle that we stand behind him," he said.
Sparks said he thinks Howe is innocent based on the lawyer's testimony in court and his good reputation in the community.
At the trial, Howe testified he had consensual sex with the woman, who cannot be identified under a publication ban. The Crown argued the victim was impaired and never gave her consent.
The jury found Howe not guilty of administering a stupefying substance.
Chief Justice Joseph Kennedy will sentence Howe on July 30.
Several speakers outside the courthouse expressed their doubts about the verdict and the role race might have played in the case to about 70 people who attended the protest.
Sparks said he is basing his opinion on his knowledge of Howe.
"I don't think he's capable of sexual assault," he said. "He's just a modest, humble, easy going, unassuming guy. If you know Lyle personally, you wouldn't think he's capable of doing this."
Chris Hansen, a spokeswoman for Nova Scotia's Public Prosecution Service, said the Crown followed its normal standards in deciding to pursue the case against Howe, determining there was a realistic prospect of conviction and it was in the public interest to proceed.
"We applied the same tests that we apply to the other 45,000 Criminal Code charges we deal with every year," she said.
The protest at the courthouse followed the march through the city's downtown from a community centre. Marchers chanted "Free Lyle Howe" in support of the lawyer, who is not in custody.
But not everyone supported the protest. Along the route, comments were written in chalk on sidewalks in support of the victim.
Jackie Stevens, executive director of the Avalon Sexual Assault Centre in Halifax, was unavailable for an interview but in an emailed statement she said she was pleased to see the expressions of support for victims of sexual assault.
"There has been a lot of discussion today about sexualized violence and sometimes these conversations can trigger or silence people," she wrote.
"It is encouraging to see that people in Halifax are expressing their support for victims/survivors of sexual assault. The legal process is often ... difficult to go through. Statements made today through social media and on city sidewalks send a message to victims/survivors that they are not alone and there are people who believe them."
Sparks said while there wasn't overt racism in the court proceedings, he believes the justice system tends to hold a bias against black men.
"When a black person comes before the justice system, race is always a factor," he said.
"For me that's a given."
Trina Briand McCalla, Howe's mother, spoke in support of her son, telling protesters he appreciated their efforts.
"I know he would never take something from anybody that wasn't given to him," she said.
"I want to thank everyone for coming out today for a righteous cause, justice for Lyle Howe."
Howe wasn't at the event.
The Nova Scotia Barristers' Society has suspended Howe's licence to practice and appointed a lawyer to oversee the transfer of Howe's clients to other defence lawyers.