The detentions will almost certainly raise tensions in the new unity government between Robert Mugabe, the president, and Morgan Tsvangirai, the prime minister.
Police did not comment on the arrests.
Previous arrests of activists from Tsvangirai's Movement for Democratic Change (MDC) have raised doubts over the new administration.
Pundits fear any political crisis could delay efforts to rebuild Zimbabwe's ruined economy.
Donors are demanding that the 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".