Organizers for the demonstration, which will bring together at least 18 opposition parties and civic organizations, say the protest will begin at midday although police with water cannons and tear gas canisters have been deployed across the capital, Harare.
The protest will mark the first time that Zimbabwe's fractured opposition has joined in a single action to confront President Robert Mugabe's government since 2007.
Dubbed the "mega demonstration," the protest brings together at least 18 political parties and civic organizations and protesters will include veteran opposition leader Morgan Tsvangirai and Mugabe's former deputy, Joice Mujuru, according to organizers.
Usually bustling with hawkers, the capital's streets Friday were bristling with police wielding batons and tear gas canisters. Some have taken positions at the headquarters of the main opposition MDC-T party. Other police have mounted roadblocks on roads leading into the city.
Protests have become a near-daily occurrence in this southern African country ravaged by a tumbling economy and widespread food shortages. Friday's protest could be the biggest and most significant yet, particularly because it could bring together Zimbabwe's squabbling opposition amid talk of a coalition to fight Mugabe in elections scheduled for 2018.