More than 3,000 Iranians living in the UK marched in London on Saturday to urge Britain to list a number of groups as terror organisations.
It comes amid rising diplomatic tension between the UK and Iran over the seizure of tankers.
The National Council of Resistance of Iran held the protest to call on the UK to impose sanctions against the Iranian regime.
It is urging for the listing of the Iranian Revolutionary Guards Corps, the Iranian Ministry of Intelligence and the Iranian supreme leader Ali Khamenei's office as terrorist entities.
NCRI’s president-elect Maryam Rajavi told the demonstrators: “Your demonstration in London today is the culmination of a series of Free Iran protest rallies that ran from Brussels to Washington, Berlin to Stockholm in a month and a half.
"It is the continuation of the annual gathering of the Iranian Resistance held two weeks ago."
Mrs Rajavi said Iran "does not understand a language other than the language of power and determination" and called on Britain and Europe to stop “paying ransom to the mullahs”, not to help reduce sanctions, and to designate a number of groups as terrorist organisations.
She specifically called on the new UK government to support human rights in Iran and to act to dispatch an international investigative delegation to visit regime prisons and to meet political prisoners, especially women.
"Today our response should be firm, held on the principle that the Revolutionary Guards and the regime of Iran should be held to account for their atrocities," she said.
Matthew Offord, an MP, backed her calls for the groups to be listed.
"These organisations should be proscribed in their entirety under the Terrorism Act,” he said.
"We should say we want to see regime change in Iran ... to achieve a better Iran and a better world order.
"The decision by world powers to decouple human rights and terrorism from the nuclear deal was a mistake and missed opportunity."
He has called for a firm response from the UK government to the tanker incidents.
"Our response must be firm and result in a firm, principled policy towards the regime that hold its leaders and IRGC to account for these atrocities," he said.
The US has already designated the IRGC as a terrorist organisation.
Opposition politician Roger Godsiff MP said: "The UK should recognise the right of the people of Iran to change the ... dictatorship and support the NCRI, its president-elect Madame Rajavi and her 10-point democratic platform for future Iran because it provide a clear road map for the Iranian people to establish a free and democratic Iran."
This week the UK announced that its warships will accompany all British-flagged vessels through the Strait of Hormuz in a change of policy after the government earlier said it did not have the resources to protect every ship from Iranian aggression.
Tension between Tehran and London has worsened since the IRGC seized the Stena Impero, a Swedish-owned tanker sailing under the British flag, in the Strait of Hormuz.
The seizure was in response to British forces apprehending the Iranian oil tanker Grace 1 earlier this month off the coast of Gibraltar.
