'Deliberately attacking civilians can never be justified,' said Amnesty International. 'Targeting worshippers during religious observances betrays complete contempt for the most fundamental principles of humanity.'

Nine coordinated attacks took place in Karbala and Baghdad on Tuesday morning as millions of Muslims were marking 'Ashoura, the holiest day in the Shi'a Muslim calendar.

'To the extent that these bombings are part of a widespread or systematic attack on the civilian population of Iraq in furtherance of an organisation's policy, they would constitute crimes against humanity. As such they would be among the most serious crimes under international law.' said Amnesty International. 'These attacks must be stopped immediately and those responsible must be brought to justice.'

Over the past few months, hundreds of Iraqi civilians have died in suicide and other attacks by armed groups. Amnesty International has been calling on armed and other groups to spare the civilians from any attacks.