Britain was accused of orchestrating bomb attacks in Iran at the funerals of six people killed in attacks that Iran has blamed on London.
State television showed mourners chanting "Death to Britain" as they carried the coffins of those killed in Saturday's twin blasts in the southwestern city of Ahvaz.
President Mahmoud Ahmadinejad, who won June elections pledging a tougher foreign policy and a crackdown on corruption, said on Sunday that Iran was "very suspicious about the role of British forces" in the bombings.
No one has claimed responsibility for the bombings, and local government officials denied reports that one suspect had been arrested and confessed to receiving training from Britain.
Britain has condemned and denied any involvement in the blasts, which were the latest in a spate of bombings and unrest in the oil-producing Khuzestan province which borders southern Iraq and where ethnic Arabs constitute the majority of the population.
The homemade bombs were left in rubbish bins outside a busy shopping mall and detonated three minutes apart, wounding almost 100 people and damaging several cars and shop fronts.
Security forces have rounded up five cells of opposition groups who had confessed to "receiving support, equipment and terrorist training," by British forces in Iraq, the Revolutionary Guards commander of south Iran said. He added that all those under arrest would be tried and executed.