Gunmen have opened fire on Shia Muslim pilgrims in Baghdad, killing at least 20 and injuring 300, officials say.
Tens of thousands are making the annual pilgrimage
Tens of thousands of Shias are making the annual pilgrimage to the tomb of revered Imam Musa Kadhim in the Kadhimiya neighbourhood.
Officials said the gunmen were thought to be Sunni extremists. Shia militiamen are guarding some of the pilgrims.
At the pilgrimage last year almost 1,000 people died in a stampede sparked by rumours of suicide bombers.
Iraq's health ministry told the BBC the deaths occurred in incidents in different parts of Baghdad on Sunday.
The Iraqi prime minister earlier warned that those using mosques to provoke sectarian violence would be prosecuted.
Security had been heightened and a ban on cars introduced to try to prevent sectarian violence.
Police said there were protected corridors for pilgrims but the sheer weight of numbers led some to try short cuts, where they were ambushed by snipers.
Militiamen protected some of the pilgrims in Baghdad
Defence Minister General Abdul Qader Jassim told Reuters news agency 30 suspected militants were arrested and 14 police injured.
Qassem al-Mussawi, head of joint operations at the prime minister's office, said there had been some shooting but the situation was "under control".
Armed Shias of the Mehdi Army militia could be seen protecting some of the route.
Some Sunni leaders accused Shias of provoking the attacks and said security officials allowed militiamen to attack Sunni homes.
"We demand that the government stand up to these saboteurs," said a statement from the Sunni Islamic Party.
The numbers at the pilgrimage remained huge despite the threat of attacks.
Men, women and children, many in traditional Shia black, carried Korans and colourful flags to the imam's tomb. Some beat their chests in a traditional show of grief.
Worshippers inside the mausoleum kissed the tomb of the imam to remember his martyrdom in the 8th Century.
One pilgrim, Kareem Risan, who had walked from the southern city of Amara, told AFP news agency: "It took us seven days. We risked our lives and braved all possible danger to reach here."
The BBC's Mike Wooldridge, in Baghdad, says the challenge now is protecting so many thousands of people on their way home.
Last year mortars were fired at the mosque housing the tomb and rumours spread of possible suicide bombers.
In the ensuing stampede, almost 1,000 pilgrims died - the highest death toll on a single day since the 2003 war that toppled Saddam Hussein.