Special security measures have been introduced in Baghdad ahead of a Shia pilgrimage that last year saw nearly 1,000 killed in a stampede.
Thousands of Shia worshippers are gathering in Baghdad
Checkpoints, body searches and a two-day ban on vehicles are in place ahead of the annual tribute to revered Shia leader Imam Musa Kadhim.
Last year pilgrims were trampled or drowned in the Tigris after rumours of a suicide bombing sparked panic.
On Friday night, seven pilgrims on their way to the ceremony were killed.
Police said the worshippers were killed by gunmen in a car, while walking to the imam's shrine.
'Death squad' arrests
Tensions between Sunni and Shia Muslims remain high in Iraq's capital.
The US military has deployed thousands of extra troops onto the streets of the capital in recent weeks as part of a new security drive ordered by Iraq's Prime Minister Nouri Maliki.
Meanwhile, US and Iraqi forces arrested three leaders of an alleged death squad said to be involved in the killing of 41 Sunnis in Baghdad's Jihad district in July.
The men were arrested during raids in south Baghdad's Rasheed district, US officials said.
One of the men is suspected of being the leader of the 9 July attack, where militants set up fake roadblocks, identifying Sunnis, dragging them from their vehicles and shooting them.
Iraqi Maj Gen Adnan Thabit said the vehicle ban would begin at 2100 local time (1700GMT) on Friday and last for two days.
Thousands of Shia pilgrims are converging on the city to mark the martyrdom of the 8th Century religious figure Imam Musa Kadhim.
Many more are expected to make their way across Baghdad on foot to the mosque housing the imam's tomb.
However, just two of the bridges across the River Tigris will be open during the pilgrimage, Gen Thabit told the AFP news agency, as authorities attempt to control the movement of people throughout the city.
On 31 August last year, mortars were fired at the mosque housing his tomb and rumours spread of possible suicide bombers.
The eventual death toll of almost 1,000 was the highest death toll in a single day since the war to topple Saddam Hussein.
Many of the dead were women and children.
At Friday prayers in Baghdad, one cleric urged pilgrims to show the bombers that their crimes would not divert Shias from their path, says the BBC's Mike Wooldridge in Baghdad.
Pilgrims are also being encouraged to protect themselves, including avoiding any sectarian talk that could encourage violence, our correspondent adds.