The Iranian foreign ministry has sent a warning message to the US, criticising its policy in Iraq.
Previous demonstrations outside the British embassy have been violent
A spokesman said Iran wanted "the departure of the occupation forces as quickly as possible and the restitution of authority to the Iraqi people".
Iran, a Shia republic, is worried by fighting in holy cities such as Najaf and Karbala in neighbouring Iraq.
Around 400 people also took part in a protest against the Iraq occupation outside the UK embassy in Tehran.
Demonstrators chanted slogans demanding the closure of the UK embassy and expulsion of the British ambassador from Iran.
About 100 of them repeatedly tried to rush the police lines but they were heavily outnumbered by the security forces, the BBC 's Jim Muir reports from Tehran.
Some stones and firecrackers were thrown at the embassy compound, which has become a focal point for demonstrators angered by what they see as the violation of Shia holy places in Iraq.
But there was more violence at a demonstration on Wednesday, when petrol bombs were thrown.
Sympathy for militant
Iran issued its warning through the Swiss embassy, as its diplomatic ties with the US have been severed.
The foreign ministry said Iraq's Shia should take their lead from their senior religious leaders.
Our correspondent says that while Iran has officially thrown its weight behind the moderate cleric, Ayatollah Ali Sistani, some hardline elements appear to have a good deal of sympathy for the younger and more militant Moqtada Sadr.
Spying allegations denied
In a separate development, Iran has denied that it received confidential information from Ahmed Chalabi, a senior member of the US-appointed Iraqi Governing Council.
However, Iran did acknowledge it had a "continuous and permanent dialogue" with Mr Chalabi, who faces allegations of passing US secrets to Tehran.
Mr Chalabi was previously tipped for high office by senior figures in the Pentagon, but has fallen out of favour since reports on Iraq's weapons of mass destruction provided by his informants have proved to be unfounded.