Tsvangirai and Robert Mugabe, the country's president, formed a unity government in February after months of political wrangling.
But frequent arrests of activists from Tsvangirai's Movement for Democratic Change (MDC) have raised doubts about the future of the new administration.
Demand for reform
Analysts say they fear any political crisis could delay efforts to rebuild Zimbabwe's ruined economy.
Donors are demanding that the country's new unity government carry out political and media reforms before they can disburse money for Zimbabwe's economic recovery programme.
Zimbabwean journalists want the unity government to scrap media laws, which critics say Mugabe has used to muzzle his opponents.
Mugabe's party, the Zanu-PF, adopted regulations seven years ago imposing registration conditions for private news organisations.
They also bar foreign journalists from working in the country and authorise almost routine arrests of journalists accused of reporting "falsehoods".