About 4,000 Iranian clerics and theological students burned US and British flags on Monday in the holy city of Qom to protest the presence of Western forces close to Shi'ite Muslim shrines in southern Iraq.
Another group of around 70 clerics and students also protested outside the British embassy in Tehran, which has become the focal point for anti-war protests in Iran.
They called on Iranian authorities to close the embassy to protest Britain's participation in the US invasion of Iraq.
The protests were peaceful, unlike one outside the British embassy 10 days ago when several windows in a building inside the compound were smashed by stones hurled by protesters.
The embassy, struck last Monday by a truck laden with extra fuel, was surrounded by heavy security, including riot police.
Municipality workers set up concrete blocks to close an opening on either side of a bus lane that passes in front of the embassy wall.
In Qom, the centre of Shi'ite Muslim learning in Iran about 125 kilometres south of Tehran, protesters chanted "Death to America," "Death to war-mongers" and burned a coffin covered by US and British flags.
Shi'ite Muslim Iran has given repeated warnings to US and British forces not to damage sites in the southern Iraqi cities of Karbala and Najaf which are home to some of the most sacred shrines for Shi'ites.
"The occupation of those cities by infidels is condemned and is an attack against Shi'ites in the world," Ahmad Khatami, a mid-ranking conservative cleric, said in a speech in Qom.
"America and Britain, by ignoring all the international rules, are acting like Hitler," the ISNA students news agency quoted Khatami as saying.
All Iran's religious schools halted classes on Monday to stage protests against the war in neighbouring Iraq, the official IRNA news agency said. Radical religious students and clerics have called on Iran's Supreme Leader Ayatollah Ali Khamenei to allow them to go to Iraq to protect its Shi'ite holy sites.