Scuffles broke out on the floor of the meeting as the protesters were challenged by angry members of the Movement for Democratic Change (MDC), which is led by Morgan Tsvangirai, the prime minister in the power-sharing government.
The MDC later said that one of its councillors had been badly hurt in the clashes.
"Gilson Chitakunye, today sustained serious head injuries after he was brutally assaulted by Zanu-PF thugs who violently disrupted the All Stakeholders Constitutional Conference in Harare," a statement said.
Tensions were high even before the opening of the meeting, with supporters of Zimbabwe's two political rivals sang songs denigrating each other.
Mugabe and Tsvangirai formed a unity government in February in a bid to end deadly political violence that erupted after last year's disputed presidential elections.
Under the power-sharing deal, Zimbabwe is to draft a new constitution that is to be brought to voters for approval in a referendum next year, paving the way for fresh elections.
Both Mugabe, who had been scheduled to open the conference, and Tsvangirai had failed to show up for Monday's meeting before it descended into chaos.
Nelson Chamisa, an MDC spokesman, said that Tsvangirai was meeting with Mugabe to discuss the disruption of the meeting by "well co-ordinated Zanu-PF cadres".
But late in the evening there was no sign of proceedings resuming and most of the delegates had left the venue.