By Frances Harrison
BBC News, Tehran
A leading Canadian-Iranian intellectual, Ramin Jahanbegloo, has been arrested at the airport in Tehran.
Reformist cleric Mohsen Kadivar (l) has expressed concern about the arrest
A prominent dissident cleric described Mr Jahanbegloo's arrest as "the height of lawlessness and insecurity".
At a gathering to celebrate International Press Freedom Day, Mohsen Kadivar said Mr Jahanbegloo was one of Iran's philosophical journalists.
The arrest has caused concern about freedom of speech as Iran comes under increasing pressure from abroad.
Mr Jahanbegloo is a well-known Canadian-Iranian professor with doctorates from the Sorbonne and Harvard University.
He has written and edited more than a dozen books on philosophy and political science.
He was arrested at Tehran airport several days ago and reports say his house was then searched.
A reformist newspaper quotes the judiciary saying there are several charges against Mr Jahanbegloo which will be made public at the end of his interrogation.
His arrest has worried human rights activists like Mr Kadivar, who himself has spent time behind bars.
"Of course I'm concerned," he said.
"In a country fighting for respect of law and freedom of press for more than 100 years, still we have a well-known figure who's arrested without a proper court order or open trial, and they don't even announce that he's been arrested. This is the height of insecurity and lawlessness."
Mr Kadivar was speaking at an awards ceremony for journalists in Tehran which honoured the writer Akbar Ganji, who has recently been released from jail.
Mr Ganji urged Iranian journalists not to self-censor or keep their newspapers open at any cost.
And he questioned why in Iran there is no real debate on the nuclear issue, saying everywhere else in the world, civil society groups protest against nuclear power plans, but in Iran everyone lines up to defend them.