Demonstrators protested outside the US embassy in London on Saturday after attackers fired rockets at a refugee camp for an Iranian exile group in Iraq, killing six people and wounding more than 40.

The attack was at a former American military base near Baghdad, known as Camp Liberty, which houses opponents of the regime in Iran.

Members of the Iranian community in Britain gathered outside the US embassy, calling on the US government to aid in the transfer of the 3,100 Iranian dissidents in the camp back to another location in Iraq, Camp Ashraf, where they were previously.

Lord Carlile QC spoke on behalf of the British parliamentary committee for Iran freedom, a 120-strong cross-party group of MPs and peers which supports the Iranian opposition.

He said: "We should demand that the UK government takes immediate action to bring this matter to the UN security council and takes steps to ensure that the residents of Camp Liberty are given humanitarian aid and returned to Camp Ashraf.

"It is time for weakness to cease in the diplomatic approach to Camp Liberty. The Iraqi ambassador should be summoned to the Foreign Office immediately."

Camp Ashraf was home to Iranian dissidents in Iraq for 26 years and is said to be 80 times bigger than Camp Liberty.

Amnesty International said the attack was "a despicable crime", and authorities in Iraq should urgently investigate and ensure all those wounded receive appropriate medical care.

The investigation should also look into the conduct of Iraqi security forces before and during the attack, and whether they failed to prevent it, a spokesman said.

"The attack against Camp Liberty is a despicable crime," said Hassiba Hadj Sahraoui, deputy director of the Amnesty's Middle East and North Africa programme.

"Authorities in Iraq must ensure not only that those responsible for this attack are brought to justice but that those living in the camp are protected."

The UN high commissioner for refugees, Antonio Guterres, expressed his shock at what had happened.

"I strongly condemn this attack," he said, noting that the residents of Camp Liberty are asylum seekers undergoing the refugee status determination process and thus entitled to international protection.

He added: "This is a despicable act of violence. I call on the Iraqi government to do everything it can to guarantee security to the residents. The perpetrators must be found and brought to justice without delay."