The latest police action comes as the opposition Movement for Democratic Change (MDC) accuses President Emmerson Mnangagwa of adopting the heavy-handed tactics of his predecessor, Robert Mugabe who died on Sept. 5.
On Sunday, Mnangagwa defended his record in an opinion piece carried by CNBC Africa, saying his administration was opening up political and media space.
But police have this year banned several MDC gatherings, saying they feared the events would turn violent.
Hundreds of police blocked roads leading to MDC headquarters in Harare but supporters continued to gather, singing and chanting before the arrival of party leader Nelson Chamisa, who was set to address them.
A few minutes after Chamisa entered the party building, police charged the crowd with batons and fired tear gas, causing a stampede, according to Reuters witnesses.
Police officers declined to comment.
MDC officials said the skirmishes once again showed that the opposition party was a victim of government brutality.
“The MDC strongly condemns that violent attack by the police on the citizens who had peacefully gathered outside our (headquarters). This kind of barbaric brutality is totally unacceptable in Zimbabwe,” MDC national spokesman Daniel Molokelo said in a statement.
Political tension is rising in Zimbabwe, where the population is grappling with a severe economic crisis that has seen rolling power cuts lasting up to 18 hours a day and shortages of foreign currency, fuel and medicines.
Most public sector doctors have been on a strike over pay since September, which has paralysed government hospitals where the poor seek treatment. Other public sector workers are demanding U.S. dollar-indexed salaries to protect them from soaring inflation.
Critics say Mnangagwa has failed to keep promises he made during last year’s election campaign to revive the economy by pushing through economic reforms, attracting foreign investment to create jobs and rebuilding collapsing infrastructure.
Reporting by MacDonald Dzirutwe; Editing by Giles Elgood.