Waving green and red flags and marching to a drum beat, men, women and children -- many of them barefoot -- flooded into the capital from across Iraq, defying threats and vowing not to be intimidated by Sunni insurgents.

Picking their way past tanks, armoured vehicles and gun-toting soldiers, the masses descended on the Imam Kadhim shrine in Kadhimiyah to kiss his tomb as the whirring blades of US helicopters drowned out the wailing of muezzins.

Old men and young boys, dressed in black and barefoot, whipped themselves with chains in self-flagellation, drawing blood in pain for the mourning of their seventh iman, who was poisoned in a Baghdad prison 12 centuries ago.

Pressing against each other, men and women dressed in flowing black robes beat their breasts in unison, pushing children in buggies draped with shawls to keep off the heat and stopping to quench their thirst with ice-cold water.

Ali Karim al-Daraji, 19, who works in a paper press and set out at daybreak, had first to fend off a tantrum from his young bride petrified that he would be killed after only five days of married life.

"I wanted to do the pilgrimage as every year but my new wife strongly objected. She cried and said 'I don't want you to die after five days in a car bomb and I won't even be able to find your corpse'.

"I told her 'Kadhim will protect us' and my mother tried to tell her everything would be fine ... Now I'm back peacefully," he told AFP.

Kadhim was the seventh of the 12 Shia imams and died in Baghdad in 799. Every year since the US-led invasion toppled executed dictator Saddam Hussein, who banned the event, Shias have marched in pilgrimage.

Well-wishers set up stalls offering refreshments to weary pilgrims, as the faithful re-enacted Kadhim's final moments, dressed up in colourful guards' outfits to pull a wooden prison cell through the streets.