At least 25 people have been killed and dozens injured in a suspected mortar attack on a mosque near the troubled city of Najaf, say hospital officials.
Kufa is a stronghold of radical cleric Moqtada Sadr
The compound of the mosque in Kufa was packed with people about to go to Najaf where Iraq's top Shia cleric has now arrived in a bid to end fighting there.
In another incident, gunmen opened fire on marchers heading to Najaf from Kufa, reportedly killing at least three.
It is not known who is behind either of the attacks.
Accompanied by thousands of his supporters, Grand Ayatollah Ali al-Sistani has travelled to Najaf to try to end the stand-off involving Shia fighters led by a radical cleric, Moqtada Sadr.
The rebels have been battling US-led forces for three weeks.
Before Ayatollah Sistani arrived, Iraq's interim Prime Minister, Iyad Allawi, ordered his forces to observe a 24-hour ceasefire in the city.
The ayatollah, who returned to Iraq on Wednesday after medical treatment in the UK, is expected to announce an initiative to resolve the crisis as Moqtada Sadr's men are holed up in the compound of the Imam Ali mosque, Shia Islam's holiest shrine.
"I have come for the sake of Najaf and I will stay in Najaf until the crisis ends," Ayatollah Sistani said on Wednesday.
Ayatollah Sistani was instrumental in brokering an earlier ceasefire between Mr Sadr's fighters and US-led forces in the city.
Hussam al-Husseini, an aide to rebel cleric Moqtada al-Sadr, told the Associated Press news agency that one mortar shell hit the mosque itself in Kufa, and two others landed near the gates of the compound.
He said another mosque in the city had also been hit by mortar rounds.
It is not clear who might have been responsible for the attack.
US-led forces said they have not carried out any operations in Kufa for 24 hours, CNN television reported.
TV pictures showed dozens of wounded men lying on the ground amid pools of blood or being ferried to Kufa's
Crowds of angry people are reported to have built up around the gates of the hospital where casualties were taken.
"We were gathering outside and inside the mosque preparing to head to Najaf when two mortar shells landed,
one inside the mosque and the other on the main gate," said one man who was taking an injured friend to the
Ayatollah Sistani has brokered peace in Najaf before
"This is a criminal act. We just wanted to
launch a peaceful demonstration," Hani Hashem said.
In the other incident, witnesses said a crowd of 2,000-3,000 were forced back from a road-block on the main route to Najaf by heavy gunfire over their heads.
The ayatollah set out from the southern city of Basra early on Thursday for the eight-hour journey in a convoy of cars and buses packed with Shia Muslim faithful.
Crowds heading to Najaf include Shia who seek the peaceful solution preferred by their spiritual leader Ayatollah Sistani, as well as supporters of Moqtada Sadr.
The Kufa mosque was regularly used by Moqtada Sadr
Aides said the ayatollah's proposals included weapons-free zones in both Najaf and Kufa - a stronghold of Moqtada Sadr - and the replacement of foreign troops by Iraqi police.
Helicopters hovered above the stream of vehicles headed to Najaf, while armed guards in sports utility vehicles protected the ayatollah.