Speaking three days after Iran released on bail the last of nine arrested British embassy staff, he voiced concern at the clampdown by Iran after the hotly disputed June 12 presidential election.
"We have to be concerned if individual rights of citizens are being affected," he told reporters.
Referring to the embassy staff arrests and to the expulsion of two British diplomats last month, he said: "We have had to make it absolutely clear to the Iranian regime that this is not acceptable."
The re-election of hardline President Mahmoud Ahmadinejad triggered a wave of street protests in the Islamic Republic, which died down after a crackdown by security forces.
Then last Friday defiant opposition supporters held fresh protests in Tehran, after powerful cleric Akbar Hashemi Rafsanjani called for detainees held in a post-election crackdown to be freed.
Brown underlined the importance of basic rights including freedom of expression.
"We've … had to make it clear that we respect the rights of freedom of speech, and freedom of assembly. And it's important that the world tells Iran that that is how we feel. And that is exactly what we've done," he added.
Iran has repeatedly accused the West, and Britain in particular, of stoking unrest following the disputed June election. Tehran expelled two British diplomats last month, triggering tit-for-tat expulsions by London.
