"The government of President [Emmerson] Mnangagwa is accelerating the repression of legitimate and peaceful activism in the run-up to the summit," said HRW's Africa director, Allan Ngari.
Zimbabwe, a country that borders Mozambique, will receive the heads of state of the Southern African Development Community (SADC) in Harare, the capital, on August 17.
According to HRW, police have detained dozens of people in recent weeks, including opposition leader Jameson Timba, and disrupted opposition rallies.
Jameson Timba, who assumed leadership of the main opposition group, the Citizens Coalition for Change (CCC), following the dismissal of previous leader Nelson Chamisa in January, was detained in June, along with more than 70 activists, during a police raid on his home.
The detainees were accused of disturbing the peace and participating in an illegal assembly.
At a bail hearing, lawyers for the detainees accused the police of using excessive force, firing tear gas into a private residence and beating and torturing their clients.
Bail was refused and the defendants are awaiting trial.
According to the human rights organization, police also disrupted a commemorative event in honor of an opponent killed in 2022, detaining several people in the process.
HRW also called on SADC to take a stance against repression.
"SADC must promote respect for human rights by calling on the Government of Zimbabwe to end repression and arbitrary arrests and prosecutions of opposition activists and supporters," said Ngari.
"The widespread climate of intimidation and repression must end," he urged.
The Government was contacted by the France-Presse (AFP) news agency, but refused to comment on the accusations.
In 2023, the opposition in Zimbabwe faced a wave of arrests following the contested August 23 elections.
The Zimbabwe African National Union-Patriotic Front (ZANU-PF), the party in power since the country's independence in 1980, has long been accused of "stifling" the opposition.
