"In the course of the past days and as a consequence of illegal and violent encounters with [people protesting] against the outcome of the presidential election, a number of our countrymen were wounded or martyred," Mousavi said on his website on Wednesday.
"I ask the people to express their solidarity with the families ... by coming together in mosques or taking part in peaceful demonstrations."
Protest in Tehran
Tens of thousands of Mousavi supporters staged a protest in Tehran for the fifth straight day on Wednesday, despite the authorities' ban on opposition gatherings.
They marched towards the Vali Asr square, protesting against what they say was a rigged election.
Many were wearing green, the colour of Mousavi's campaign, but Al Jazeera's Alireza Ronaghi, reporting from Tehran, said he had spotted a new trend in their outfits.
"I saw that it was becoming a prevalent trend to wear black ribbons as well, which is a sign of mourning in Iran, a sign of sympathy for the victims who have died in protests the days before," he said.
Political protesters apart, a dozen Iranian journalists and bloggers have been arrested in the aftermath of the contested presidential election, according to Reporters Without Borders.
The government has put restrictions on foreign media coverage in Iran after the election, and authorities accused some foreign media of being the "mouthpiece of rioters".
"Some countries, in an uncalculated, hasty and rude reaction towards the illegal gatherings, have supported them contrary to democratic principles and regulations and have become the mouthpiece of the rioters' movement", the foreign ministry said in a statement on Wednesday.
Several internet sites such as Facebook, Twitter and YouTube have been blocked and the Revolutionary Guard, an elite military force, warned the country's online media users they will face legal action if their uploads "creates tensions".
Al Jazeera's correspondent said the move against websites and blogging by the Guard was beyond their remit.
"Their move to crack down on websites and blogs is against their constitutional rights, but they see things spreading out of hand, so they feel it necessary to intervene at this point," Ronaghi said.
"It is obvious that the Revolutionary Guard does not see itself as a pure military organisation.
"They have been telling the media and activists that the Revolutionary Guard was founded as a political and military foundation of the Islamic Republic, so they see it as appropriate to enter politics whenever they deem fit."
Within the country, mobile phone text services have been down since the election
Violence on tape
Despite these measures, violent scenes of police beating Mousavi supporters taken on mobile phones have been broadcast on news bulletins across the world.
At least seven people have been killed in recent clashes between the authorities and the opposition movement, according to state media reports, while hundreds more are thought to have been injured.
Among those arrested by the authorities since the protests against the presidential election results began are several reformists.
|Iranian police have badly beaten some protesters at the opposition rallies [AFP]
Hamid Reza Jalaipour, a sociologist, was arrested at his home on Wednesday morning, Issa Saharkhiz, a colleague, told the AFP news agency.
Saeed Laylaz, a political and economic analyst, was also arrested at his home by four officials, a family member said.
Jalaipour and Laylaz are also prominent journalists.
The Guardian Council, Iran's most senior legislative body, said it could order a partial vote recount, provided it finds irregularities.
The council ruled out annulling the disputed poll, the main demand of the opposition.
The foreign ministry summoned the Swiss ambassador, who represents US interests in Tehran, on Wednesday to protest at "interventionist" US statements on Iran's election.
President Barack Obama told CNBC there appeared to be little difference in policy between Ahmadinejad and Mousavi.
"Either way we are going to be dealing with an Iranian regime that has historically been hostile to the United States," he said.