Supporters of Iran's defeated presidential candidate Mirhossein Mousavi are preparing for a day of mourning for eight people killed in mass demonstrations against what he says was a rigged election.
Iran's English-language state television has reported eight people killed in protests since official results from Friday's poll showed President Mahmoud Ahmadinejad had been re-elected.
His huge lead on Mousavi, a moderate former prime minister, has provoked Iran's worst unrest since the 1979 Islamic revolution. Bloodshed, protests, arrests and a media crackdown have rocked the world's fifth biggest oil exporter, embroiled in a dispute with the West over its nuclear programme.
Security agents have detained opposition politician Ebrahim Yazdi while he was in hospital, an ally of his said.
Yazdi, who heads the banned Freedom Movement and was foreign minister in Iran's first government after the revolution, was among scores of reformists rounded up since a firestorm of protest ignited over the disputed election.
On his website, Mousavi called on Iranians to demonstrate peacefully or gather in mosques wearing the colour of mourning - black as opposed to the green of his election campaign.
"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," he said.
"I ask the people to express their solidarity with the families ... by coming together in mosques or taking part in peaceful demonstrations."
Ahmadinejad defended the legitimacy of the vote, telling a cabinet meeting on Wednesday that it had "posed a great challenge to the West's democracy," Mehr news agency reported.
"The ideals of the Islamic Revolution were the winners of the election," Ahmadinejad said, adding that 25 million of 42 million voters had approved the way he was running the country.
Yazdi was detained while he was in hospital having checks for stomach problems, a member of his Freedom Movement said.
There was no immediate comment from the authorities.
Yazdi, who is a prominent opposition voice but has limited popular support, told Reuters on Monday the election had exposed deep schisms in the leadership of the Islamic Republic, which he described as facing its biggest post-revolutionary crisis.
The authorities reject charges that they rigged the vote, but scores of thousands of Iranians have braved riot police and religious militia to show their anger on the streets, ignoring Supreme Leader Ayatollah Ali Khamenei's call for national unity.
Khamenei is due to lead Friday prayers, an occasion expected to be used as a show of strength by Ahmadinejad supporters.
The protests of the last five days have laid down a direct challenge to the authority of Khamenei, who has traditionally stood above the daily political fray.
"His prestige has received a blow," said a senior Western diplomat in Tehran, who argued that Khamenei's call for unity had clearly failed to settle the showdown with the opposition.
"Now the lines are drawn. He must give more to the opposition otherwise this will continue," the diplomat said, adding that he still expected a compromise that would preserve Iran's hybrid system of clerical rule and limited democracy.
Iran has denounced foreign criticism of the election, even though US President Barack Obama's administration has muted its comments to keep the door open for possible dialogue.
The Iranian Foreign Ministry on Wednesday summoned the Swiss ambassador, who represents US interests, to protest at "interventionist" US statements on the country's election.
The White House denied meddling, but said Obama would continue to defend the right of Iranians to protest peacefully.
"The people of Iran reserve the right to have their voices heard and their votes counted," said US Secretary of State Hillary Clinton. "It is for the Iranians to determine how they resolve this internal protest" over the election outcome.
Mousavi sent a letter to Iran's state national security council complaining of plainclothes agents using sticks, metal rods and sometimes firearms to "attack the lines of peaceful participants before the arrival of the security forces".
He condemned arrests of his allies in the past few days.
The ISNA news agency, quoting provincial officials, said 88 people had been arrested in post-election unrest in the conservative northeastern city of Mashhad and up to 60 people in Tabriz in the northwest, where Mousavi comes from.
Pro-Mousavi protests were also reported in the cities of Isfahan, Rasht, Orumiyeh, Zanjan and Zahedan.