Hundreds of thousands of Shia Muslims are thronging the streets of the central Iraqi city of Karbala for the climax of a religious pilgrimage that was banned for a quarter of a century.
Self-flagellation is one of the rituals of the pilgrimage
The Shia, who form the majority in Iraq, are using the occasion to celebrate both their faith and the end of Saddam Hussein's rule, correspondents say.
As many as 500,000 Shia have descended on the centre of Karbala, chanting as they pass the shrines sacred to their faith, while many more are filing down the main roads that lead to the city.
Some shouted anti-US slogans as they flocked to the centre, demanding the withdrawal of coalition forces from Iraq.
In Baghdad, thousands of people have been continuing an anti-American protest which began on Monday when the new US administrator of Iraq, Jay Garner, visited the capital.
The pilgrims in Karbala are flocking to the shrine of the Prophet Mohammed's grandson, Imam Hussein, to commemorate his death in a 7th-Century battle between Muslims over who should be the rightful successor to Mohammed.
Shia Muslims mourn Prophet Mohammed's grandson Imam Hussein
They are marking 40th day after his martyrdom in battle at Karbala in 680 CE
Battle sharpened split between Sunni and Shia Muslims
Mourning includes self-flagellation and chest-beating
The BBC's Damian Grammaticas in Karbala says local people hosed the pilgrims with water to cool them after days of walking.
Each group had a banner bearing the name of their home town or village - almost every one of which across central and southern Iraq was represented.
Some men had faces streaked with dried blood where they had slashed their heads with knives and many pilgrims sang and beat their chests, our correspondent says.
Correspondents say some Washington officials are unsettled by the fact that what is moving to fill the vacuum created by the collapse of Saddam Hussein's regime appears so emphatically Islamic in character.
Many Shia clerics have been urging their followers to take to the streets and demand the immediate withdrawal of coalition troops.
On Monday, protesters gathered outside the Palestine Hotel in Baghdad complaining that a Shia leader, Sheikh Muhammad al-Fartusi, and other clerics had been arrested by US forces.
Within hours of the protest, it was reported that the cleric had been released, although US military officials never confirmed that he had been detained.
BBC News Online's Martin Asser, who watched the protests, says further disillusionment with the coalition could cause problems, as US plans for post-Saddam Iraq would be near impossible if Shia co-operation was withdrawn.
Shia groups are already threatening to boycott a major civic meeting on Thursday being organised by Mr Garner.
The retired US general, who insists he is Iraq as a "facilitator not a ruler", spent his first day assessing some of the damage war had inflicted on the capital, where many basic services remain crippled.
On Tuesday, he travelled to northern Iraq, where he held talks with Jalal Talabani, veteran leader of the Patriotic Union of Kurdistan.
Correspondents say the reception he received contrasted markedly with that on his arrival in Baghdad, where he faced anger over the lack of water, electricity, and law and order.
Mr Talabani gave Mr Garner a warm welcome, despite on Monday making it clear that he objected to any "foreigner" leading an administration for Iraq.
"For you it is your home place. When you retire come back to Kurdistan...and we'll
prepare a beautiful house for you."
It was Mr Garner's first visit to the region since he ran Operation Provide Comfort, co-ordinating humanitarian aid after Saddam Hussein crushed a Kurdish rebellion following
the 1991 Gulf War.
Mr Garner's mandate is to install an interim administration before Iraqis form their own government and regain control of their country.