Demonstrators protested outside the US Embassy in London after assailants 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 facilitate the transfer of the 3,100 Iranian dissidents in the camp back to another camp in Iraq, Camp Ashraf, where they were before.
Lord Carlile of Berriew QC, 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, said: "We should demand that the United Kingdom 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.
Scotland Yard said that the protest at the embassy was peaceful.
The United Nations High Commissioner for Refugees, António 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. "This is a despicable act of violence."
He added: "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."