Those who violated the election process "stood beside the main instigators of the recent riots and shed people's blood on the ground", Mr Mousavi said, pledging to show how they were involved.
Mr Mousavi, a former prime minister, spoke of the "recent pressures on me" that are "aimed at making me change my position regarding the annulment of the election".
He described the clampdowns he and his staff were facing.
"My access to people is completely restricted. Our two websites have many problems and Kalameh Sabz newspaper has been closed down and its editorial members have been arrested," said Mr Mousavi, who has not been seen in public for days.
"These by no means contribute to improving the national atmosphere and will lead us towards a more violent atmosphere," he added.
Day of mourning
Opposition leaders had called for a day of mourning on Thursday, but some reports say it has been cancelled.
Separately, nearly two thirds of MPs appear to have stayed away from President Mahmoud Ahmadinejad's election victory party.
All 290 MPs were invited to attend the party, Iran's press reports, but only 105 turned up. An earlier BBC report wrongly reported that 105 did not attend.
One of those who reportedly failed to turn up was Parliament Speaker Ali Larijani, a high-profile figure who shares some of Mr Ahmadinejad's hardline views but has been critical of some aspects of the government's handling of the protests.
About 50 MPs are reformist and would not have been expected to attend the victory party.
12 June Presidential election saw incumbent Mahmoud Ahmadinejad re-elected with 63% of vote
Main challenger Mir Hossein Mousavi called for result to be annulled on grounds of electoral fraud
Street protests saw at least 17 people killed and foreign media restricted
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