The G8 summit is being held in the Italian city of L'Aquila
Prime Minister Gordon Brown has praised his fellow G8 leaders for their "strong" condemnation of Iran for detaining a UK embassy employee.
In a joint communiqué, the leaders attacked "arbitrary restrictions on or intimidation of" embassy employees.
In a separate statement, US President Obama and Mr Brown said the detention of embassy staff was "unacceptable".
Mr Brown said the UK wanted a "strong statement on Iran and this is a strong statement from the whole of the G8".
More than 1,000 opposition supporters and prominent reformists were reportedly arrested in the aftermath of Iran's controversial presidential election, and three weeks later it is possible hundreds remain in prison.
Authorities in Iran are also holding a local employee from the British Embassy, who has been charged with "acting against national security".
On the second day of their summit in the Italian city of L'Aquila, the G8 leaders said they were "seriously concerned" about the situation.
The statement added: "Interference with the media, unjustified detentions of journalists and recent arrests of foreign nationals are unacceptable.
"Embassies in Iran must be permitted to exercise their functions effectively under the Vienna Convention without arbitrary restrictions on or intimidation of their staff."
It also said the leaders were "deeply concerned" over Iran's nuclear programme.
Another joint statement from President Obama and Mr Brown, issued by the White House, said: "The President expressed solidarity on Iran, where both leaders were clear that the detention of Embassy staff by the Iranian authorities was unacceptable"
Mr Brown said: "I am grateful for the solidarity of the other leaders."
He added that the UK was prepared to support France as it called for the release of French teacher Clotilde Reiss who has been detained for a week in Iran in what Paris says are allegations of spying.